totally stopping the fertilizer is not a good idea. you need to cut back. there are no rules to fertilizing, many of my ponds need far less fertilizer than is recommended. when using liquid fertilizer during summer months my usual application rate is one quart per acre and see what happens.
I recently purchased some fertilizer from Southern Excellence. its advertized in Pond Boss, its the fertilizer that floats in its own bucket. it works very well and the best thing about it is if the pond gets over fertilized you can simply remove the bucket. there is not another fertilizer on the market that you can take out after you put it in. thats sweet. I'm thinking about going to it full time.
As far as your O2 crashes, remember warm water holds less oxygen than cold water. if you test for oxygen in the winter you can expect 14 ppm, test the O2 level in the summer and you will get 7 ppm, thats the way it is. I would recommend that you cut back on the amount that you are fertilizing, let the secchi disk be your guide and give fertilizer at least 2 weeks after application before applying more. Phytoplankton is a weird plant and it does not always do what you think it should. Usually I'll add light doses of fertilizer and see what happens, you can always add more.
Or you can consider a feeding program, that usually takes the place of fertilizer nicely in bass/bluegill ponds. the problem with stopping the fertilizer is the fish population will usually crash. it greatly reduces the amount food available to your fish. if you are certain that you dont want to fertilize anymore than you need to consider harvesting some of the fish from the pond. that will help too.
