only thing about the sand is with the cichlids which are notorious diggers they stir up the sand and it tends to clog the filter. not all that familiar with the flounder if he is a digger or not. what size tank are you putting him in?
i have an inch of gravel and half inch of sand in the choc zebbie tank but the L002 and the ancistrus soon dig up the gravel underneath should have stuck with less gravel and more sand ( only put in the gravel for the plants)
Decided to go for some ultra fine multi-coloured gravel (my daughters were with me so they chose the colour) The loaches and the floumder seem to like it anyway. Only drawback is my water is very very cloudy due to taking the old gravel out and putting the new stuff in. O well its slowly clearng up, should be ok by tomorrow. (I hope)