I have a similar problem with a 1/2 acre pond - everything disappeared except GSF. You might try trapping the GSF if you're in a hurry to reduce their population. I set out two size traps (homemade), one for very small fry and one for larger size fish. These are funneled floating rectangular traps made with pvc and plastic mesh and can be baited with floating catfish pellets. After only a couple of hours, I trapped at least 30 GSH and 2 water snakes in the large trap and more than 200 in the fry trap (far to many to actually count).
