Assuming enough O2 the FH will do fine. Even better if you throw them a few pellets once in a while. FH do fairly well in low DO (as compared to LMB). DW and WM have a survival method called turions. They are like dormant seeds in the pond bottom. They stay there until an event kills off the surface plants allowing light or warmth to the bottom. At that point they float up ( buoyed by it is believed CO2 from anaerobic respiration) and start the process all over. That is why you often have to treat more than once with a systemic product that stays in the water a while. You need to put some spawning structure (pallets for example) for the FH to spawn under in shallow water (3 ft or less).