Before adding anything else I would send a sample of the water to Texas A&M University to get it tested. You might find that there is enough nutrients in the water already but something else is limiting the phytoplankton bloom.

It might be alkalinity and not pH that is limiting the bloom.

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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).