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My 40 gallon bare bottom fry tank has about 35 three week old Discus still eating powder fry food four times a day. Room temp runs 72F and mid tank reads 82F but the back corner feels cooler near the sponge filter intake. I do 30 percent water changes twice weekly with matched temp dechlorinated water and dim lighting about eight hours daily. Worried a submersible heater will cook fry against the glass or swing hard after changes. Inline on the sponge line or skip heater and heat the room instead? No tankmates in there yet.

For fry that young I would skip heating the whole room and run a fully submersible heater with a guard or put it horizontal low near the sponge filter outflow so circulation mixes the heat. A 75 watt on a 40 gallon is plenty if you are holding 82 to 84F. Inline heaters on sponge lines are sketchy at fry size because flow is low and you get dead spots that spike locally. After your twice weekly 30 percent changes pre mix new water in a bucket with the same 82F setting and float the heater in the bucket five minutes so the swing is maybe half a degree not three. Point the sponge outflow across the tank not into a corner. Check with a digital thermometer at gravel level since bare bottom loses heat fast. If fry hug the glass by the heater dial back one degree.

Yeah that corner chill is real, my sponge filter side always runs cooler too. I run a 100 watt guarded sub on low in a 36 inch fry tank with bare bottom and like 40 fry on powder food four times daily. Keep it horizontal under the outflow not vertical on the glass. Match bucket temp to tank before those 30 percent changes and they stop stacking in the warm spot. I do dim light eight hours same as you and temp stays 82 to 83F after I stopped heating the room. No tankmates either till they size up.

Inline on a sponge line sounds clean but low flow means hot pockets before the fry even notice. I use a 75 watt horizontal sub in a 30 gallon bare bottom fry tank with about 50 two week olds on powder fry food and micro worms every other day. After a 25 percent water change I float the change water bucket with a spare heater ten minutes, same trick Dodge mentioned. Fry quit hugging the back corner once I aimed the sponge outflow across the middle. Room is 70F, tank holds 83F, lights on seven hours dim. If they pile on the heater guard drop to 82F, no big drama.

What size heater did you buy already if any? I run a 50 watt in a 20 gallon fry tank with sponge filter only, bare bottom, five week old Discus on beef heart mix once daily and powder food twice. Room stays 68F and tank is 84F stable after I stopped doing big 50 percent changes cold. Wondering if 100 watt is overkill for your 40 gallon with only sponge filtration and no canister yet.

Picked up a 100 watt submersible but have not plugged it in yet. Fry tank is still 40 gallon bare bottom, sponge filter only, three week olds on powder four times daily. Last water change was 30 percent Tuesday with bucket matched to 82F. The unheated corner still reads 79F with my digital probe while mid tank is 82F. Should I run the new heater at half setting first or go full 82F target right away? Also thinking about adding a second small sponge filter on the opposite end for flow before I install anything.