Stocking a 40-gallon breeder tank

Why a 40 breeder stocks differently

I bought my first 40 breeder used, with a crack in the stand that I found an hour after the fish went in. The tank itself was fine and I still run it. It is the size I'd point most people to once a 20 or 29 feels small.

The numbers are 36 by 18 by 16 inches. That 18-inch depth is the difference. A 29 has a 30 by 12 floor, which is 360 square inches. The breeder has 648. Cories, kuhlis, rams, and plecos live on the floor, so that extra area counts for more than the extra 11 gallons. The tank is short, though, which is bad news for tall fish like adult angelfish.

Each plan below is built around one centerpiece fish or one kind of tap water. I checked every list in the stocking calculator at 40 gallons. None of them pick up a warning, and the first one fills the tank to the limit.

Plan 1: Bolivian ram pair over cardinals and cories

Two Bolivian rams, 12 cardinal tetras, 8 peppered cories, and one bristlenose pleco. Keep it at 76 to 78°F on fine sand with driftwood and a couple of caves. The shared temperature range for these four is 73 to 78°F, so a warm 80°F discus-style setup is out because of the cories.

This plan uses nearly all of the tank's waste budget, so it has no room for later impulse buys. If you want slack, drop the bristlenose and wipe the glass yourself. Rams pair off and guard a flat stone when they spawn. A 36-inch tank gives the cardinals and cories somewhere else to go.

To go cooler or skip the rams, swap them for one angelfish and the cardinals for 10 rummy-nose tetras. One angel with 10 rummies and 6 cories also passes the calculator. Run that one at 76 to 78°F as well.

For an algae crew in place of the rams, use three Siamese algae eaters, 12 harlequin rasboras, and 6 peppered cories. That list also passes the calculator at 40 gallons. Keep a tight lid on it, since SAEs jump.

Plan 2: Pearl gourami trio with harlequins and kuhlis

One male and two female pearl gouramis, 12 harlequin rasboras, and 6 kuhli loaches. These like it warmer, about 77 to 81°F, with soft to moderate water and low flow. Floating plants and tall stems give the gouramis cover at the surface. Sand lets the kuhlis dig.

Pearl gouramis have the least room for a second male. Two males in a 40 will spend the day flaring. Kuhlis hide all day for the first few months. Count them after lights out with a flashlight rather than tearing the scape apart.

Plan 3: Hard-water swordtails and platies

One male swordtail with three females, 6 platies, and a bristlenose pleco. This is the plan for hard, alkaline tap water with a pH of 7 to 7.8. Keep it at 72 to 78°F. Swordtails use the full 36 inches. The tank depth also gives you room for plant thickets where fry hide.

You will get fry, and plenty of them. Set up a second tank, ask your local fish store whether they take fry before you have 40 of them, or keep only females. Keep the tetras from plan 1 out of this one. They want softer water than these fish.

What does not belong in a 40

An oscar needs 75 gallons or more. A common pleco grows past a foot long. Two fancy goldfish already go over the calculator's waste limit for a 40, before you add anything else. Two angelfish plus a full school of tetras and a cory group also go over.

Avoid one of everything too. Eight different species in groups of two or three makes a tank that is crowded and full of stressed schooling fish. Pick one plan and fill out its groups properly.

Filter, heater, and stocking order

Use a filter rated for at least 40 gallons: a Fluval 207 canister or an AquaClear 70 hang-on-back. Add a 150W heater for a normal room. Change 10 to 12 gallons a week. If nitrate reaches about 20 ppm before the next change, change more water or cut the stocking.

Cycle the tank before adding fish. Add the bottom group first, then the school a couple of weeks later, then the centerpiece fish once ammonia and nitrite stay at zero. The rams, gouramis, or male swordtail go in last, so they settle into a tank that already has fish in it instead of claiming all of it.