Aquarium pH spiking a lot?

I cannot get my aquarium to stabilize. It's been a month and I have a three step external filter. The filter has a sponge layer, activated carbon layer, and a bio layer. I did a 25% water change a couple weeks ago with filtered water that had a pH of a little over 7. Everything stabilized, then my betta got sick again. He's the indicator of something wrong, I guess. He got sick last time I had aquarium problems. So I tested my levels and ammonia, nitrite, nitrate are high. I don't know what to do. Am I overfeeding? Is the tank overcrowded? I have 38 gallon with 19 fish. 1 betta, 8 tetra, 4 mollies, 4 dannios, and a pleco. How much should I feed them?