The first step in determining what is causing the decrease in your fish population is to test the water parameters in your tank. If your tank is not properly cycled, the ammonia and nitrite levels may be too high, which can be lethal to your fish. You should also check for other signs of stress, such as pH levels and temperature. If you find that the water parameters are all within acceptable ranges, then the next step is to observe the tank inhabitants. If you are seeing the tadpole madtom hunting the other smaller fish, then it is likely the cause of the population decrease. You may want to consider adding more hiding spots so that the smaller fish can have a safe place to hide from the tadpole madtom. Additionally, you could try to separate the tadpole madtom from the other fish in the tank by using a divider or a breeder net, so that it cannot hunt the smaller fish.My tank is well-planted and seemingly happy, but the fish population has dropped in the past year. I'm sure natural attrition explains much of that, but I'm concerned the tadpole madtom (about 3 inches long) is eating the least killifish specifically as their population has gone from "hordes" to "a few", and no other fish would seem capable of eating them. Same thing happened to the bluefin killifish population. Other fish in the tank include several golden topminnows, 2 female sailfin mollies, a couple bluefin killifish, about 3 mosquitofish, 5 flagfish, and a tiny swamp darter... nothing that would seem likely to eat anyone else.
I've seen some articles saying TMs only eat invertebrates, algae, and detritis, not fish. However, he is tough to get food to as he hides all day, so I wonder if he's resorted to nighttime raids on bigger prey. Anyone know for sure if this guy is likely to be picking off the little fish?
I hope not... my 10g NPT has a thriving population of amphipods and snails (only) and I really do enjoy watching them... and I know they'd be quickly consumed if I moved him there. Hope I'm not forced to build a 3rd tank ;D