Two days ago I bought 3 tetra longfins and placed them in a 10 gallon tank that I had going for a day. It has a filter and a water heater set at 75 F (and I notice they always hang out by the heater). It seems like they don't eat the flakes before they hit the ground, and there are many flakes that end up going to the ground because of the filter. What should I do to make sure that the fish eat enough?