I’ve seen tanks around 160 liters (about 40 gallons) that house more than 30 large ranchu and oranda goldfish. The water remains crystal clear, odorless, and the fish are healthy. When it comes to koi, filtration systems in Asia are often half the size of the pond itself, and some even build filter chambers equal to two-thirds of the pond’s volume. That’s why seeing large koi crowded together in clean, clear water is quite common here.

Even with discus fish—known for being extremely sensitive to water quality—Southeast Asian breeders are masters at developing new, colorful strains. Our discus tanks are stocked very densely, yet the fish stay strong and healthy. Therefore, stocking density primarily depends on the filtration system. What may seem overcrowded and risky for one person could be perfectly fine for another. It doesn’t mean that the latter doesn’t care for their fish or doesn’t know what they’re doing.

Things won’t be too crowded. That assumes you have adequate filtration and aeration.

I would like to add one grown Sevrum.
Is that going to be too crowded?

My parameters are stable.

You are hunting for nano fish. Good choices include schools of six or more: Daisy’s blue ricefish, white cloud mountain minnows, otocinclus, pygmy corydoras, Celestichthys margaritatus (“celestial pearl danio” or “galaxy rasbora”), glowlight tetra, dwarf pencilfish, various small danios, small barbs such as ruby, and cherry, and small rasboras such as emerald, dwarf, lampeye, chili, phoenix, somphong, galaxy and pygmy. A school of green kubotai rasbora is stunning! Note ALL these fish get aggressive if not in schools of at least six fish

Look up the adult size of the fish. Some experienced nano fish enthusiasts recommend sticking with fish under one inch as adults. The list of fish which get larger than one inch includes such fish as the harlequin rasbora (2 inch), pork chop rasbora (2 inch), golden barbs (4 inches), Odessa barbs (3 inch), checkerboard barbs (2 inch), most corydoras (3 inch), black phantom tetras (2 inch), lemon tetra (2 inch), bleeding heart tetra (3 inch), tiger barbs (3 inch), and most of the glofish (the danios are fine). Hobbyists will be surprised how big these fish get. They MAY be cramped in small tanks. But this is an OPINION, not a scientific fact. So if you want a school of harlequin rasboras in a ten gallon, go for it!

Other good fish which do not need to be in schools are Endler livebearer, scarlet badis (Dario Dario), dwarf chain loaches, peacock gudgeon, sparkling gourami, Tanganyikan shellies, and the betta. My personal favorites are the Tanganyikan shellies (Lamprologus ocellatus and their relatives). They are tiny cichlids with great personalities. Just be sure to include some two inch snail shells which the “shellies” use for homes.

Bettas are good but generally need to be alone in a small tank (sometimes they tolerate tankmates and sometimes they do not). Avoid blackwater fish like neons, cardinals, rummy nose tetras and rams. These are NOT starter fish. Same goes for dwarf frogs and shrimp. Read this article for more information. http://tankstartguide.com/index.php/17-9-nano-aquarium/

I want to add some fish, either soon or after the aquarium can be considered cycled, I don’t mind. What would your recommendations be? I prefer small fish, as it is a small aquarium, and that would allow me to have more of them; I am also a beginner, so easy fish are preferred. Thank you very much for your help

In reply to M …. Yes the black ones. Stuff the HOB with as many of those inserts as will fit. And you can replace kuhli’s with Cories.