Step 1: Main Element Is a Poultry Water Feeder
The main device that supplies the makeup water is a plastic poultry water feeder. It works on a principle of a vacuum forming at the top of the reservoir which prevents water from flowing out until air is allowed to enter. In our case the air enters from a hose end that opens to the air as the fish tank water level reduces. These can be purchased at an Ag Store or on the Internet. The one we used holds 5 gallons and has a removable base which will not be used. Remember it must have an air light lid. This one uses a rubber gasket. The yellow arrow shows the 3/8 inch plastic hose barb with 3/8 inch pipe thread that was screwed into the water outlet. Made threads in the heavy plastic outlet with a 3/8 inch metal pipe plug. Put silicon over the surface to assure there would be no leaks.
Step 2: Fish Tank Water Outlet
A 3/8 inch plastic hose is used to get water from the reservoir to the the tank. The water level will be determined by the outlet position of the hose so it must be held securely. Made this one from some stainless sheet metal with Nylon screws to hold it to the tank and small stainless screws to hold small plastic pipe clamps that support the hose. Inserted a small piece of ridged plastic pipe into the flexible hose so it could be clamped securely. The Nylon screws at the bottom were cut to a length that kept the stainless sheet metal properly positioned in the tank.
Step 3: Fixture Holds Reservor in Correct Position
Tried just placing the water reservoir in an upright position but found the water would not flow out ! Believe it was due to surface tension in the hose. Tried an number of hose positions etc but nothing worked. Found this approach was best. Built a fixture from wood to position the reservoir such that the outlet is facing down at about a 20 degrees. Kept the hose straight to the "tank hose holder" with no dips. Worked great. The water will stop at the fish tank water surface as air can not get back up into the reservoir to reduce the vacuum that is holding it from flowing out.
Step 4: Reservoir in Place in Fixture
This is a photo of the reservoir in place in the fixture. Note it must be placed above the fish tank. The outlet in our case is about 1 foot above the tank. But it could be further away if needed. If there is no shelf nearby it could be placed on a ladder since it is only needed for a vacation etc.
The reservoir is filled with water then lifted in position while holding the tank "hose holder" and hose above the top to keep the water from spilling.
Step 5: Hose Holder in Place in Fish Tank
This is a view of the "hose holder" in the fish tank. We kept it at a depth so the water was about about 1 1/2 inches from the tank top. When we returned from vacation there were about 2 gallons of water drained from the reservoir and the water level was just where is was when we left!
Since that section of the aquarium lid could not be closed due to the hose holder location we made a plastic cover for this section to reduce evaporation and to keep the fish from jumping out!
Step 6: Automatic Feeder
Not part of the water replacement system but a key part of returning to live healthy fish after 2 1/2 weeks is a way to feed them! We have used this feeder for many years. It comes with a holder that can feed fish for several weeks. It's also adjustable for the number if times per day they can be feed. We use it everyday to feed at about 2 PM. For this long trip we set it up to feed twice per day. All 6 Discus were happy and healthy on our return! We used a mixtured of dried and freeze dried food.