Riyhone

Hi - I am fairly new to keeping fish, but things were going well, tank had cycled, no ammonia/nitrates etc. I have a 90 litre tank, about 14 cardinals, 4 catfish, 4 clown fish, 2 mollies, 3 plecos, 5 tetras. A few weeks ago, I started getting ammonia spikes after changing a filter. I got a lot of advice, told to change bio noodles to a different type. Anyway, for the last fortnight, I have had to do water changes every day. Ammonia is shooting over 2ppm daily, and even though I ensure neutral pH at water change, the water gets very acidic very quickly. So much so that in the last week, I have noticed it goes off the acidic chart after a few hours. It seems the whole system is getting the wobbles very badly. I went a day without any feeding and no change in this pattern. I know I don't overfeed, if anything I underfeed at the moment. Due to ammonia being present, I have started using ammonia lock as recommended by a fish shop. I wonder if I have managed to kill all the ammonia eating bacteria. I use Stress coat when changing water, and Stability (seachem). Anyway - help! what do I do now? Fish still seem OK but I can't keep doing this every day...I am not touching the carbon filters or Eheim substrat Pro at all. Seems fish shops give plenty of advice but nothing has helped. Why does my water go from pH neutral to off the chart acidic so fast? Why won't the ammonia come under control?