Since the tank will have no top and there will be various streams in the bog area, evaporation will be rampant. There are auto top-off systems in the catalogs I frequent, but they seem overly complicated, monitoring sump level, tank level, reservoir, etc. I found a system on Ebay that looked promising. It comes with one or two float valves that can be configured in a variety of ways depending on your needs. It activates a relay to a standard 3-prong outlet. As a safety measure, you can set a timer for between 6-24 minutes. This is how long it will run the pump before shutting off, whether or not the float said to stop. This protects against floating plant matter interfering with the float valve and overfilling the tank. If it times-out in this way, it won't run again until it is reset.
I opted for the two float system. This gives you a float for on/off and a "oh sh#t" backup. If all else fails, there is the timer.
You may have noticed a blue plastic 55 gallon drum next to the 150g tank. This is my tank water queue. Although I upgraded my reverse osmosis system to better support my new tank needs, it still doesn't produce water at a fast rate. So I have it setup to fill the tank to a mechanical float valve. Then I have an electric pump that I use with a garden hose and spray wand to fill my other tanks. For the 215 I added a two-way splitter, routing one to the hose, and the other to a lawn sprinkler valve with a hose barb. I run hose from there to the tank. I leave the hose above water level so the water can't flow backwards into the drum when the water level in it is low. The transformer to power the valve simply plugs into the controller for the float switches. The switches came with plastic brackets and suction cups, but I plan to build a more permanent mount for them.