way back in the sixties I established a tank in the sunset old folks home...it was enjoyed by the residence soooo much...unfortunatly tooo much ..each and every oldie would give 1/2 there biscut to these fish every day untill the staff asked for it to be removed...it used to have biscuts floating all over the place..my sagestion is sort out the lid so they cannot feed them.
then again there is a tall hex tank in arran court where my mum is & it is fine..maybe too high for them. it has about 6 coloured goldfish in it so reduces the power bill I guess...all the best.