I've got a 350 gallon pond stocked with goldfish. Pond has been established for over 5 years but is experiencing mini cycles due to my father keeping it with little fish keeping experience. Question is would a 250 gph filter be sufficient? I'm aware that aquariums need 8-10x gal for gph. Having some issues with ammonia, nitrite and nitrates. Ammonia running 0.5 ppm, nitrite 0.25 ppm and nitrate 80ppm. Doing daily water changes at this point but the fish are in bad condition. Most are missing scales and some have even changed color due to stress. I'm thinking the filter is not sufficient at 250 gph for a fully stocked 350 gal pond?