In reply to Stefano Salt buildup is one of the legitimate uses for water changes. A 50% change very two months should prevent any problems.

thanks!

In reply to Strefano ….. just good old tap water.

Thanks for your time!

In reply to Alpacaphobic …. Hydrogen peroxide is deadly to some fish. It doesn’t break down into simply oxygen and water. It breaks down into a form of monoatomic oxygen (Hydroxyl radical) that kills any cell it comes in contact with. The guppies somehow got a full blast of it and died. The Flourish had nothing to do with it. I won’t use any algaecide including peroxide simply because they ALL can kill fish.

Filtration-wise, it has the HOB, a double-headed sponge filter, and another large sponge filter, all mature. It had one of the good HOBs that hold a tone of media, but the motor burnt out when we had a power outage a few days ago. I replaced it with a brand-new and totally inferior HOB, but on the day of the water change disaster I had switched that out with an HOB identical to the old one, that had been running in another planted tank with pea puffers for five or six months.

So, then I did a water change of maybe 50%. I run tap water, dose it with 4-5X Prime, let it sit in buckets for a few days. Out of the tap it doesn’t even register on the chlorine tab on my test strips, pH of like 7.8 and TDS of ~200, GH and KH register in the middle range on said test strips.

After the water change, I added a full dose of Flourish, might have been a little more than my tank needed, but not by much. Maybe 20 minutes later, I hit a rock covered in black algae with hydrogen peroxide. Big rock, lots of algae, used maybe 3 of those turkey baster pipette things full of H2O2.

I walk away, do some dishes, come back a half hour later and all of the adult guppies are clumped up at the top of the water next to the sponge filter outflow.

I scoop out as many as I can into a running quarantine tank where I’d been scooping out extra fry anyway, and do a water change that probably ended up being like 90%, and drop in an extra sponge filter and a big-ass bubble stone for good measure.

I should mention that through all of this, the platies never even acted like anything was off. Neither did the ghost shrimp, which I’ve kind of considered canaries in my tanks, they’re the first to kick off if something is wrong.

Anyway, one of the oldest males was already dead on the bottom of the tank when I noticed the problem, and the other full-grown guppies followed, after spending a few hours at the top of the tank, or huddling under the filter. Then the juveniles went. Then the fry.

Even the fish that had been scooped out as soon as I saw them didn’t make it, so it had to be an immediate injury, whatever it was. A few of the fry had some very weird seizure-looking spells before they died, just twisting and flopping frantically in the middle of the water column, then straightening out and going on their way!

And this morning of course I wake up to a tank full of green water, in what had been my most stable and trouble-free tank!

The only thing I can think of is possibly some weird reaction between the hydrogen peroxide and the Flourish, which looks like it has more iron than any other active ingredient? The way they were huddled at the most oxygenated water in the tank obviously points to an issue with O2, right? Like, as I understand it the peroxide just turns into water and oxygen, but would the oxidation reaction with the iron and/or other crap in the Flourish end up depleting the water’s O2? But how would a temporary (I’m guessing) depletion of O2 cause so much damage to only the guppies that they died in perfect water hours later, while the platies and ghost shrimp (GHOST SHRIMP OF ALL THINGS) are just hanging out, being not dead?!?!

AARRGGGGHHHHHHH. This is killing me — obviously I did something wrong which I should not do again, but what?!?!????