Under tank wet/dry filters

[Scubaman]

I'm just wondering about all those filters that go underneath the tank. What happens if the power goes out and therefore the pump doesn't return the water back to the tank? Will the system overflow, possibly flooding the room and creating a big problem?
I'm thinking of making my own and I'm really worried about this.
Thanks.

[vjvl51]

Scubaman, whether you use a sump or a wet/dry, they both work on the same principle.

Let's start at the tank. The water exits the tank one of two ways. If your tank is predrilled (usually in the bottom) - this is sometimes called a reef ready tank - there is usually an overflow area. This overflow is really just a partition that separates the water in the tank from the water going down the pipe to the sump. This partition usually has slots to allow the water to flow over and into the drain section. The water in the main tank will not drain any lower than those slots (usually about 1 inch). Once the water gets into the overflow area, it just drains out via gravity.

If you don't have a reef ready tank, then you must use a syphon which is basically a (sometimes 2) "U" shaped tubes that connect two boxes - one inside the tank and one outside. Again, this syphon will only drain the tank by about 1 inch.

Sometimes people use a syphon tube without the inner box. Then they drill a small hole in the syphon tube about 1 inch below the water surface. When the water gets below that hole, air enters the tube breaking the syphon so no more water comes out.

Now, let's move to the sump or wet/dry. Your pump runs all the time (we hope :)) pumping water back to the tank. If water stops draining into the sump, the pump will empty the sump and then burn out due to lack of water. Usually before this happens, the tank overflows onto the floor. If you are using the syphon method of emptying your tank, just check daily that a snail or debris has not clogged your syphon tubes and this shouldn't happen.

As the pump pumps water into the display tank, the water level rises causing water to spill into the overflow or the syphon to drain water from the tank. From this you can see that the pump controls the speed at which the water drains.

Now if the power goes out, the pump does not raise the water level in the tank so the tank will drain to the level of the slots on the overflow partition or the syphon break hole. Your sump or wet/dry has to be able to hold the amount of water that will drain out of the tank and pipes. In my case, the water level in the sump raises about 2 inches so I make sure that the water level in the sump is never less than 3 inches from the top of the sump. It is always a good practice to check this (by turning off all pumps, including the skimmer, etc) after every modification to your system's plumbing.

The thing that many people forget is that your pump will automatically act like a syphon as soon as the power goes out. Make sure that the end of the hose is less than one inch under the water or that you have a syphon break hole about 1 inch below the water surface. Part of your weekly maintenance is to ensure that this hole has not become clogged by anything or you could end up draining more water out of your tank than you planned on. :(

Vickie

[AquariumPro]

Hi:

Vickie covered it all. The other thing you can do to prevent back-siphoning through the pump is to install a check valve on the pump. Please post if you have any other questions and I'll be notified.

[MJC]

Scuba,

This is all assuming your sump or wet/dry have the capacity to accommodate the overflow.

Let's say your tank is 60x20x24 (150 gallons) and the overflow box or tube removes approximately 1" of water to your sump or wet/dry. This equates to approximately 6.25 gallons of water in your wet/dry or sump. Your wet/dry or sump will need to be able to accommodate this extra 6.25 gallons.

A sump is basically a container for extra water and how much volume you have in when it's "online" should be less than the aforementioned 6.25 gallon. Assuming a 25 gallon sump (36x12x13), you could in theory have as much as 17 gallons of water in it before your system goes "offline" and contain the additional 6.25 gallons or as little as 3 gallons before your pump burns out.

With a wet/dry filter, you need less water to aerate the bio balls, preferably at one to two ball diameters or less, IMO more water in your wet dry (during operation) lessen's the filter effectiveness.

Assuming again a 25 gallon wet/dry, you'd need a minimum of 4 gallons to a maximum of 12 gallons to keep the filter running. As your system goes offline, there'd be more than enough capacity to take up the extra water.

HTH,

Marty