Where did the 5 glo fish, 6 neons and 2 Danios go?, so you added 5 cardinals , and 2 platys? That's 20 fish if you lost 9 like you stated in a previous post that's still 11 but you wouldn't be overstocked at 11, you also stated you only do water changes every month which is bad because you're not replacing the minerals the fish need, water left unchanged the minerals break down.
So I'm kinda confused where did the other fish go? did they all die and you started over? With all new fish?
The 50 is fine for that tank I run a 110 and a 70 on my 55, no you're not overstocked but it's recommended you do 14% water changes weekly on aqadvisor, but i would still do 25%.. Here's the thing when you don't test your water..
One week you could have a spike , the next everythings good because its starting to build bacteria, then it spikes again to finish the cycle, the next week its good, you keep saying "sometimes I don't see ammonia" ... You should never see it as a well maintained tank will have 0 ammonia and 0 nitrites at all times.
I would find a way to take 30 min a week (the time it takes me to manually do my water change no python) on a 55 gallon with lots of decor /drift wood /plants /rocks and do a pwc every week, I would not wait took your api conditioner is out I would go with prime as pretty much everyone in her has mentioned to you it is the best.
This might of came off in the wrong way but I assure you I'm just trying to guide you in the right direction so you don't have so much issues.
As far as the lid missing Yes it could be the issue, if you clean around the tank with chemicals they can get inside the tank, my buddy lost all his fish from painting the room the tank was in because of the vapors I guess, I don't know how that works as paint didn't get into the tank, do you have any chemicals by the air pump? Because the pump could suck in the vapors and put them into the tank, when you clean the outside if the tank you use a clean rag and just water right, never use anything to clean the glass with, just wipe it down with a clean rag no window cleaner nothing just water.