do not use an antibiotic to kill bacteria in your tank. antibiotics do not differentiate between good bacteria and bad.
you can use a product called "chemiclean" by boyd enterprises. follow the instructions exactly.
ultimately you have excessive nutrients in the tank. i don't care what the hobby grade test kits say. as far as flow goes, i've had cyano
grow on the outputs of power heads, so i don't think it's the lack of flow.
it's not a "cuc" , or lack thereof either. No eating, pooping, creature is going to correct the problem of excessive nutrients. it's common sense. when you eat something, it does not disappear. it just changes. the same goes for crabs and snails. they end up pooping it out.
water changes with good source water and cutting back on feeding is the best defense.