DIY Auot-top off in 4 steps

[Dale]

Mik, I can see three problems with your plans.

1. Silicone will not make a watertight seal when used on plastic or acrylic. A better method of attaching the airline tubing would be to drill a hole slightly smaller than the diameter of the airline tubing in the lid, cut the airline tubing at an angle and pull it thought the hole with a pair of pliers.

2. Water in two containers connected by a tube (as you've suggested) will always try to find equilibrium. There is a name for this law of physics but it escapes me at the moment. This means the water in the jug will flow non stop without concern for evaporation. As suggested, the way to slow this is by using some sort of valve to reduce the flow rate. The problem with IV type drippers for top ups is...

3. Evaporation occurs at different rates while the dripper system is constant. Two things that effect variations in evaporation rates the most are light photo periods and room temps (which go up and down if you have a programmable thermostat).

Kent sells a float valve for simple auto top offs that retails for about $30. That (and a bracket to hold it in the sump) will solve all of your problems if you're still pursuing that route.