You've gotten some good advice already, but to highlight what I think are the main issues:
1. You are overstocked; a problem that seems to be taking care of itself. The problem is only going to get worse in a hurry since you are keeping some of the fastest growing monsters in the hobby.
2. Running a filter during the day and turning it off at night is not a good idea. The stagnant water in your HOB is basically rotting overnight and when you turn it on in the morning you are introducing all sorts of nasties into your tank.
3. In regards to the airstone, I wouldn't be worried with aesthetics when your fish are going belly up. Extra aeration can't hurt, especially if it's an issue of low disolved oxygen.
As a little experiment, can you try testing the water in your HOB filter in the morning after it's been sitting stagnant overnight? I'm willing to bet that's your issue. I bet you get some high ammonia results from the water in that HOB filter. Just be sure to dump the stale water out and refill with tank water before you restart it.
1. You are overstocked; a problem that seems to be taking care of itself. The problem is only going to get worse in a hurry since you are keeping some of the fastest growing monsters in the hobby.
2. Running a filter during the day and turning it off at night is not a good idea. The stagnant water in your HOB is basically rotting overnight and when you turn it on in the morning you are introducing all sorts of nasties into your tank.
3. In regards to the airstone, I wouldn't be worried with aesthetics when your fish are going belly up. Extra aeration can't hurt, especially if it's an issue of low disolved oxygen.
As a little experiment, can you try testing the water in your HOB filter in the morning after it's been sitting stagnant overnight? I'm willing to bet that's your issue. I bet you get some high ammonia results from the water in that HOB filter. Just be sure to dump the stale water out and refill with tank water before you restart it.
