Oftentimes there are nitrates present in tapwater.
A fully cycled tank should maintain 0 ammonia, 0 nitrite, and ever-increasing nitrate levels (unless planted...plants consume nitrates) If you've never had nitrates, then you should probably be adding a nitrogen supplement of some kind for your plants, as they aren't getting enough afaik.
After 4 months going with no problems, then yes your tank should be cycled. I'd be inclined to look at your filter at this point. Especially since you said that adding an airstone seemed to improve the condition of your fish.
Having to guess, I'd say that you have a single HOB filter on there, and it's creating a dead spot in your tank, as well as not aerating enough water. It would also account for an ammonia reading in a long-standing tank. I run two HOB filters on any of my tanks over 40G (one on each end) to make sure there is enough water movement. And if you need more aeration, the marineland emperor line is awesome
A ph of 6.0 or less is low, but I can't say it crashed if I don't know your tap water and base readings. Was the PH always low? Ottos are usually the first to go when the water parameters take a dive, poor little fellows. Are you running some kind of DIY co2 injection? I know that can drive the PH down.
It's really hard to give any solid advice without knowing the specifics of your setup.