Ah, I see you never figured out why they died, though I still believe it was due to high heat/ low oxygen.
Adding a hang on back protein skimmer would be very helpful. it super oxygenates the water, while removing wastes. Just make sure you get one with an overflow option, that can drip into a bucket if you go on vacation. Some tanks collect a cup of waste a day, some just an ounce. There are low profile models, that are 3 inches deep, since your tank is already set up.
As for an auto top off, you don't need the best one, there are mechanical models, Jebao makes one for a few bucks. You put the pump in the 5 gallon bucket, and the sensor in the main tank, but if you have current in there, the sensor wont work well. Most people place the sensor in the sump where the water is calm.
A simpler method would be a a drip system. Thats using a plastic container placed above the tank, and a simple valve on the hose, which drips water in at a slow rate, like 1 drop every 2 seconds. That adds a calculable volume of water into your tank per day. I think the container they sell is a bit on the small size, like 1 gallons, Take a look at it and you'll see you can make your own using the rectangular 2 gallon water with spigot at the super market, and tubing of the right diameter.
