Well, if the fish are going to be in the 300 gallon tank for any length of time, I'd either plan on using some sort of pond filter to help minimize the waste in the water (and ammonia) or I'd plan on doing water changes every other day or so. (and make sure the water going in is warm enough for them)

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).