[ownedbycats]

Some fish are plant/algae eater. Most pet fish are insectivore/carnivores, but there are enough common fish like plecos and catfish that need plant material in there diets that algae wafers get made. I even occasionally catch my insectivore ricefish nibbling on the edge of a wafer, but they don't eat much of it.
Some algae is inevitable, but you do want to avoid lots algae, if too much shows up it usually means something is unbalanced. Either you have too much light or it's left on too long, or leftover food (algae uses this as fertilizer), or waste from you fish, or..... (dozen more causes I haven't been told about yet)

Oh, did you find the hidden frog yet? I'll give you a hint: look in the plant.