Frozen brine is almost a guarantee of added phosphate if you haven't rinsed it with RODI
Near 0 is NOT ZERO and that is especially true for Ammonia and Nitrite. As suggested, you may want to get some fresh quality test kits for pH, ammonia, nitrite and nitrate. Seems like you may be overfeeding too. Is all the food gone in 3-5 minutes?
Follow the previous advice and cut back feeding to half of current ammounts and then only every 3 days. Make sure you put all frozen food in a brine shrimp net and rinse it with RODI
or distilled water, then feed what is left in the net.