If its covered in salvinia it will likely have to be sprayed more than once. That stuff is the DEVIL, you can spray it once and let it die back and hit it again. The problem is when it dies it takes up oxygen and will kill the fish (seems crawfish would be alright, but don't know). If you are not worried bout the fish, you may be able to knock it all out of there. It spreads so easily by boats and trailers and seems to double in coverage in a week. The weevils are not going to eat all of their food source, but may knock it back. Drawing the water down is the best if you have that option. Good luck
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