My recommendation is “natural” aquarium gravel, such as the picture above. The cream through brown varied color hides accumulated detritus and feces. Mono-colored substrate such white gravel simply look really bad when fish poop, which is something fish do ALL the time.
Small gravel, such as the 3-6 millimeter colored gravel sold in most aquarium stores, works reasonably well as an aquarium substrate. Anything over 6 millimeter (1/4ths inch) can start trapping food. Note that a mix of three to eight millimeter gravel such as most modern aquarium gravels is just fine as the small particles fill the voids in the larger particles.
Small gravel does increase the bioload somewhat (maybe very roughly + 5% to 10%) by trapping some food particles. But it doesn’t produce abrasive particles. Note that some cheaper aquarium gravels which are epoxy coated will flake off the epoxy coating in time, something which is very undesirable. White on black spotted gravel really looks bad.
Larger gravels such as pea gravel are simply bad for any aquarium. When fish are fed a small proportion of the food drops to the bottom of the aquarium. In most aquariums the food will simply lie exposed on the bottom where the fish can still get to it and eat it. With large gravel the fish can’t get to this uneaten food, it decomposes, and it creates an increased bio-load in the aquarium. I won’t use pea gravel or large decorative gravel. And don’t get me started on using marbles in the aquarium.
I never mix gravel which is very light or white with gravel that is dark. The resulting “salt and pepper” look is very unnatural. It looks like a lot of feces on the bottom of the aquarium. But then some like this aesthetic! I prefer small mixed brown gravel (light brown, medium brown and dark brown mixed together or just the “natural” aquarium gravel one can buy) and a natural look to the aquarium. Of course, some people and children prefer the psychedelic pink look to the gravel. To each his own.
Of course, 3 to 6 millimeter “aquarium gravel” is the best gravel for an undergravel filter and works well in an undergravel filter. I have had great success with undergravel filters and had them in all of my fish only aquariums.
One very misunderstood topic is using a gravel vacuum. The brown mulm which forms down in the gravel is neither good or bad. It adds a little to the bioload of the aquarium water. But it also does some very limited biofiltration. And by “biofiltration” we mean both the action of “beneficial bacteria” (more properly termed “nitrifying bacteria”) and all the other beneficial organisms found in brown mulm. So the negatives and positives pretty much weigh each other out.
But a brown mulm above the gravel is a different matter. If bottom feeding fish like corydoras can swim through the mulm it is decidedly detrimental. One constantly sees “My corydoras lost their barbels. What happened?” on social media. When they post a photo the aquarium almost always has a brown mulm visible above the substrate. This mulm must be prevented. Prevention is best done by circulating the water in the aquarium in such a way as to sweep the mulm up into the filter.
Oil Absorbent and Kitty litter
Aquarium gravel has gotten quite expensive. Two very real alternatives which are a WHOLE lot cheaper are the various sintered clay products used for Kitty Litter (typically light gray color) and clay oil absorbents (shades of rusty brown). These are CAN BE very attractive as a substrate, especially the rust colored clay oil absorbents.
The oil absorbent and kitty litter have one big problem. They typically cannot be used in an undergravel filter as they have too much in the way of fines. I’ve had good luck with oil absorbent but some of my readers have had disasters. I think it all depends on the lot number of the absorbent. The amount of fines is not closely controlled and varies bag to bag, lot to lot, and manufacturer to manufacturer.
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Hi Dave, can the Kitty litter be used for undergravel filters?
In reply to Noor …… Yes it decidedly can be used in undergravel filters. Note there is considerable variability in the amount of fines found in different brands
Thank you Dave! The bottom of tank matten also sounds intriguing. In your opinion, for a Malawi aquarium do you think it would work well? As there would be no “traditional” substrate as such in the tank.
Would you suggest putting a thin layer of kitty litter on top of the sponges so I can put some artifical plants?
In reply to Noor … A Matten undergravel filter with a half inch of kitty litter on top will work well for a Malawi Tank.
Hi Dave, thank you for your insights. Following up on the cat litter question, how well does it have to be rinsed? I bought a bag of unscented, non clumping cat litter, and when put in water, there is quite some amount of “mush” forming.
If mush forms it wasn’t completely calcined and can’t be used for gravel.
Hi Dave, Can you weigh in on the contentious “coloured gravel will leach chemicals into the water and should be avoided at all costs” debate? Perhaps this was an issue with epoxy-covered gravel in the past, but we’ve moved onto better methodology since then?
IN reply to JLaw … Not really much of anything to it. Colored gravel is safe and always has been safe. Only the “return to loincloth” folks think otherwise with no data to back up their beliefs.
Hi Dave I found a All-Purpose Granular Oil/Fluid Absorbent that contains a natural blend of diatomaceous earth and montmorillonite clay. I know the clay is okay, but the d.e.?
In reply to Brad … It should be just fine.
Hi Dave.
I’m getting mixed signals. In reply to Noor you said kitty litter can be used with under-gravel filters.
But on the planted substrate discussion (15.7) you said:
“Calcined clay products such as Kitty Litter or Oil absorbents. Very inexpensive substrates that grow plants very well. Don’t work well in under-gravel filters.”
And:
“Note that calcined clay products have too many fines to work well with an under-gravel filter. This is true even if weed blocking screening or mosquito screening is used.”
Kind of puts me in a pickle. I’m running some tests on my locally available litter and absorbents to see if they break down in water, but the tank I want to use them in is going to have a UGF with powerheads. I have some unwoven weed blocking fabric I can use to cover the filter plates, but if the dusty clay particles are small enough to either get through or clog then obviously that’s a no-go. Any clarity on the topic?
In reply to Kyle: Some key words from you “locally available litter and absorbents”. These are highly variable products. Sometimes they work and sometimes they don’t work. Based on the latest feedback I’m recommending going with expensive aquarium gravel for undergravel filters rather than trying to cheap it out with “locally available litter and absorbents”, sand or Black Diamond.
Hi Dave,
What a treasure this web compendium is. I have set up a 20g quarantine tank to establish strong bio filter media that can be used to seed the main display aquarium. Also want to plant this tank to possibly propagate some plants.
So, I used Seachem Fluroite Dark and Fluorite Black as substrate. Therein is my question.
I have an undergravel filter with bubblers in this 20 g tank along with a decent canister filled with pot scrubbers, K1 and 20 ppi foam (3 different due to the filter’s water path).
But I used Seachem Fluorite Dark and Fluorite Black. Their tech support said it was fine to use as a substrate with a UGF. But the brown fluorite dark is quite silty. Now I’m concerned and wondering if I should just remove the UGF now before the filtration has cycled.
In reply to Frank The silt will slowly work its way to underneath the UGF plates where it will be harmless. So just leave all in place and you should do just fine.
Hi Dave, just found your site. I have a 5 Gallon freshwater tank with a gravel substrate. My tap water is 7.4 Ph my tank is 8.2. I read that the gravel will raise my ph. In the past 4 years, I’ve lost 3 Bettas. I was in this hobby 25 years ago and had no problems with my fish. Got out of the hobby, because of moving. Decided to get a betta so I got a nano tank. Filter, heater. I’m lost why I’m losing these fish. I have a nerite snail left. I’m stumped. Thanks for any recommendations.
In reply to Connie …….. Bettas can be a short lived fish. Your gravel is doing nothing to the tank. Your tap water has excess carbon dioxide in it which is gassing off and raising the pH a little. But 8.2 is a fine pH for bettas. Take a long look at your feeding. Long term deaths with no clear cause often have over-feeding as the root cause.
Hi Dave, great information on here for an old/newbie trying to get back into it. I have not had an aqaurium for many years, but now I have the time to put into it. I just got a 32 gallon bow front glass tank with stand. My starting point. I see gravel is your choice. What fish would help clean the mulm and other waste best, other than the one you listed.
Also what should my next step be after substrate/gravel/etc… i want plants and at least 15 fish or other aqautic life if that is feasible. Looking at bright colors, tangs, maybe an angel or 2 . Cichlids, etc…. any help or a point in the right direction is greatly appreciated!
Whew OK you want plants. That means you need to stay away from African cichlids and Central American cichlids (they up root plants). Other than that have at it.
Hello Dave. I have four main questions for you, though I’m sure I’ll find many more.
As a side note, I love your articles. You shot down all my fears about adding driftwood and river rocks into my aquariums. I had no idea those myths were so widespread and commonly accepted as fact! Your input would be immensely appreciated. Thank you
In reply to Fanofshrimp ………
1, When the flow slows down the media needs to be cleaned, leaving 10% to 20% of the “brown gunk” behind in the filter.
2, Any dead leaf is fine. Just do not overdo it. One or two dead leaves are fine.
3, All shrimp are equally easy. Just remember they do best in a mature tank with no fish
4, No ideas here. I don’t remember ever keeping WCMM’s
Hi Dave. It is alright to use gravel from the side of a hill in an aquarium after a good rinse? I’ve found a good looking spot with gravel that isn’t too large to trap food and I am 99% positive that there are no contaminants or anything.
In reply to Fanofshrimp ……… Go for it. It should work out just fine.
Hi Dave,
Thank you for all the content. For a curious guy like me it is like a buffet. It has helped me a lot.
I am planing to build a pond in my backyard and I’m wondering what I could use for filtration media. I’m planing to use some foam or k1, but here in Brazil they are quite expensive at least for the amount I gonna need. Also there are no pot scrubbers here. I will import some as well, but the taxes here are very high. So I will need to add more filtration to have clear water.
I am thinking about using crushed stones in something like an ugf filter. I’m not sure if it’s the correct translation. Those are small stones used in construction. I’m not sure how well they would work or even if the would work. They are carbonate rocks (limestone?) so I guess it will increase the PH of the pond. After reading your section about PH I guess the PH will increase just to a certain point and I should not worry about it, right?
Also the small size ones have around 4 to 9mm and they are not round. Not sure if it’s a problem. I’m planing to add a good mechanical filtration so the stones don’t get clogged.
What your thoughts about this? Thanks a lot.
In reply to Raphael ……….. Small gravel or crushed stone in an undergravel filter set-up works quite well. I did it with a goldfish pond and the water was crystal clear. The problem comes into focus when you ask “what do I support the gravel on?”. I used lighting panels and found they broke rather easily. Between the opossums and the breakage I finally gave up.
Crushed limestone should work just fine. It will rise to around 8 pH and stay there.
And the shape of the stone should make no difference.
Thank you very much.
The water here in Brazil generally has a very low PH, so I think the gravel will actually be good.
I still didn’t decide about the filter design I gonna use. The guy from ozponds.com advocates for using a bog filter which looks like an ugf filter. He uses something like a drainage pipe to hold the stones. I don’t know if the pipe would allow the water to flow evenly. This design seems very simple and easy to build.
Here in Brazil, most of pond projects uses a kind of giant sump, generally hidden below a deck, It looks good (although they generally use bioglass), but they are more expensive to build.
To support the gravel I thought of using some kind of step plate
They look strong, but the holes seems to be quite big, so I would have to place some kind of geotextile to hold the gravel.
Well, thanks a lot again.