To add a little bit to Ken's statement...
" There are two solutions to this which should be balanced. Drop the level of nutrients getting into your pond -- like cleaning or diverting runoff water from a cattle pasture or other big nutrient source. The other half of the equation is to add enough plant life to your pond to absorb many of these nutrients. It could be floating islands, shallow water plantings, etc."
The FA comes from to much nutrients and when it dies adds back to the source. The Tilapia eat the FA, and if you catch them and take them out of the pond in the fall, you also reduce the nutrient level for the next cycle. That's a good thing. If they die, they might add back to the cycle, but most likely they will get eaten by stuff that will keep those nutrients form recycling.
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I have had tilapia for two years now, and highly recommend them.
