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Well, I've now finished the planning of my Marine project and the work has started. I've just completed the construction of a substantial cabinet to house the 48"x18"x21" aquarium. I have today placed the order for the aquarium. I have followed the advice from the recent debate about Siphon box -vs.- drilled aquarium, and opted for a drilled aquarium.

First question: I have opted for a 37mm (1.25") hole to feed the sump. Bearing in mind that I will be using an Eheim 1060 (or equivalent) for circulation, will this hole be big enough to handle the quantity of throughput without flooding? Your opinions would be appreciated as I have the opportunity to change the spec if general opinion is that its not big enough.

From: HL

I like your attitude, and the way that you are obviously researching everything. This is good, and I wish you luck with your project. Here's my view on your first question:   Once the hole is drilled, and the tank is in place, it will be very difficult to change anything. So, just because I'm conservative, I would play it safe and rather drill two holes (either on the same side, or on two opposite sides of the tank. If you do not need the second hole, you can always plug it (or even glue a piece of glass across it with silicon), but it will be easily available should you decide to increase water flow in the future, or find that the one hole can not quite cope with what you need.   The volume of water that can flow through a given diameter pipe is a function of the "head" of water, i.e. how much higher the water level in the tank is than the top of the standpipe or weir surrounding the outlet hole. The formula to calculate outflow from the base aperture of a container is as follow:

Where   Q = water outflow volume (in cubic meters per second, so multiply the answer by 1000 to get litres per second) C = "discharge coefficient", which is ~ 0.601 for sharp edged apertures, and ~ 0.941 for well rounded apertures A = cross sectional area perpendicular to the water flow (in square meter) g = gravitational constant (~ 9.8 meter per second squared) H = Water "head", i.e. height of water surface above discharge aperture (in meter)

As an example, if we assume a 10mm head above the 37mm diameter standpipe inside the tank, the maximum water flow through your proposed 37mm (ID?) hole, will vary between ~ 300 - 500 litres/hour (86 - 135 gallons per hour). If the "head" increased to 25.4mm (1"), the flow would increase to between ~ 500 - 800 litres/hour, which still is not much. Of course, if you built a larger area "weir" around the outlet aperture, as is normally the case, you could easily have a 100mm, or even a 200mm "head", which will be more than adequate to handle your proposed pumps.

I have just finished my sump for my new 390 Litre tank. The tank measures 1300x615x550 mm. I also have the left rear corner drilled for my out/overflow pipe. This leads down to my sump, situated under the tank. My sump holds a total of 60 litres. After testing the set-up with freshwater this weekend (I have just finished my sump), one piece of advise that I may be able to share with you is with regards to the outflow in the tank. I first tried it with just the stand pipe level with the water surface. This worked but not very well. The surface film of water did not move very rapidly. I threw some sawdust on top of the water and it took ages to end up going down the pipe. I then fitted a Perspex cup that I made from some 70mm diameter Perspex pipe with a piece of Perspex sheet glued on one end, drilled to fit tightly over the pipe. I fitted this on top of the standpipe and hey presto, the sawdust cleared within a few seconds. It gives the overflow a much bigger surface area to skim the surface film (I will probably have to switch off my return pump for a few minutes every time I feed the fish, otherwise everything will end up in the sump).

It does also cause a rather loud gurgling noise to emanate from the outflow. I will have to experiment with different ways to quieten this noise down. The water level inside the cup is the same as the level of the standpipe. The tank level is at the top of the cup when full, thus the cup actually forms a small weir. By doing this it helps force more water down the standpipe. (it creates a higher pressure). The sponge is there to act as a small pre-filter to prevent uneaten food ending up in the sump.

From: CK

Thanks for the reply. I'll probably use the 30 gallon (~135litre) as it fits nicely into the area I have planned. Between your suggestions and HL's design, I believe I have some good starting blocks. Is there a problem with locating the sump in a cabinet about 1 metre from the tank? I can't imagine there is any huge advantage to having it directly under the tank.

No I don't think you will have any problems if the sump is not directly under the tank. I had to place mine there because my tank stands in our living room. My wife would have had a fit if I had put an ugly Perspex box next to the tank. The only consideration with a remote sump is the choice of piping and the route you run it by. Sorry if this sounds a bit like a Mechanical Technology lesson, but by keeping a few small principles in mind it will greatly improve your systems operation.
The longer the pipe the more friction it causes and hence the less flow you will get. Try and remember this when you size your return pump. General opinion is that flow through the sump should be at least 5 x Tank volume. Try and keep your return piping the same size as the discharge port on the pump. Too small restricts the flow and too big places unnecessary back pressure on your pump. It is normal practice with pumping systems (even large industrial installations) to make the suction or incoming pipe larger in diameter than the discharge/delivery pipe. This ensures unrestricted flow to the pump and will prevent internal damage to your pump (Known as cavitation, which also causes noise). Make sure your fittings (elbows, T's, valves, etc)  do not reduce the internal diameter of the piping (especially on bends). This results in high friction losses in your piping. My return pump is rated at 1400 Litre per hour, but when I first connected it up I was not even getting half of this flow. After dismantling all of my piping (a real crappy job) I discovered that the elbow that joined my 20-mm piping to my spray bar reduced the diameter to 8mm inside the elbow. I removed this fitting and replaced it with one with a 20-mm bore. As soon as I switched on the pump it was clear to see that the flow rate had greatly improved. It is now just over 1000 Litre/hour. All that extra work for one bend.

I'm still confused about the flow rate. I can see how it works with a drilled tank but it's the theory using a siphon that I'm having problems with. If the drain has a larger capacity than the return then surely the water will flow out until the siphon break is reached. If this happens and the siphon is not started again the pump will keep returning water to the main tank until the sump is empty! Maybe I'm missing something about the design of the siphon in the main tank. If it didn't need priming after the water level reached the siphon break then I can see the flow would start again.