The flakes and tap water are contributing to your issue. Running the water through a filter isnt going to remove the stuff that causes cyano to grow, and those filters are only good for about 5 gallons. You will need to get a better source of water if you want to fix these issues.
In the mean time, switch to a quality frozen food (like rods food or marine cuisine) and only feed every other day.
And the reason your parameters look ok is that they cyano is remove the nutrients before the tests can register them. The ugly algae are extremely efficient at up taking nutrients. If you have algae issues, your water parameters are off somewhere.