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Discussion Starter · #1 ·
Hi there

I am working on a DIY background for my 125 gallon Mbuna tank. I am hiding the heaters and filter intakes behind the background. My question is can I make the holes to intake water to the pocket behind the background 1 inch in diameter or should they be bigger? One on each end that feeds an intake and heater on each end.

Thank you
Jim
 

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The short answer is, make the holes as large as you can and cover them with a screen and/or hide them in the background (i.e. create an overhang with the hole under/behind). Or, make more smaller holes. Given what I've calculated below in the long answer using reasonable assumptions for the amount of flow you might have going through the intakes to filter a 125g, I'd say you're probably going to want to make your intake holes through the background larger than 1 inch. You're probably going to want at least 2 inch holes. Keep reading for the long answer and the math that you can use to calculate the hole size. Or at least post your filter flow rate and I can give you a more informed short answer.

The long answer depends on how much flow needs to go through the hole(s) (and fair warning, the long answer is long and involves math/algebra). Then you'd want to decide how much you can allow the water level in the chamber behind the hole to drop. In order to flow through the hole, a pressure differential needs to be created and the way that is created is by having a different water level on either side of the hole. So your intake pulls water out of the chamber behind the hole, causing the water level to drop. When the water level drops, water is pulled from the tank through the hole into the chamber. The water level in the chamber will drop until a pressure differential is generated large enough to cause the flow through the hole to equal the flow through the intake. The larger the hole, the less the water level needs to drop. If the hole is large enough, the drop in water level will be insignificant.

What you have is the engineering equivalent of a submerged orifice:


The equation at the bottom is what you want to use.

  • Q is the flow you need to pass through the hole. With a filter you usually know Q in gph. This needs to be converted to in^3/s for this equation to work. [gph]*231[in^3/gal]/60[min/hour]/60[sec/min] = [in^3/s][/*]
  • C is what's referred to as the discharge coefficient and is usually .6 to .7[/*]
  • a is the area of the hole. the area of a circular hole is pi*r^2. so a 1 inch diameter hole has an area of pi*.5^2 = .7854 in^2[/*]
  • g is the gravitational constant which is 386 in/s^2[/*]
  • delta h is then how much the water level will drop in the chamber behind the hole.[/*]

delta h is your unknown and a is what you can control, your variable. The rest of the terms are constant. You can rearrange the equation to solve for delta h like below. Or, if you know what you want delta h to be, you can rearrange to solve for what your hole area a needs to be.



idk what your filter flow is but for a 125 let's say you've got 1000gph split between the 2 intakes so 500gph flowing through the 1 inch hole. Doing the unit conversions and math, the water level in the chamber behind the hole would drop 6 inches lower than the water level in the main part of the tank (using .6 for C). So that would tell you how low the filter intake needs to be or how far to submerge the heater.

Delta h is proportional to the square of the flow rate so if your flow is more than what I've assumed, delta h will increase quickly. If your flow is twice what I've assumed for example, delta h will be 4x larger (24inches). You can see that a 1inch hole quickly becomes inadequate. If flow is half what I've assumed, h will be 1/4x as large (1.5inches).

The good new is that delta h is also inversely proportional to the square of the hole area. And, hole area is proportional to the square of the hole diameter. So if you made your hole 2 inches in diameter instead of 1 inch, the area of the hole would be 4x larger, and the delta h would be inversely squared that, or 1/16x as large. So for the same 500gph flowing through a 2inch hole instead of a 1 inch hole, the delta h would only be 3/8 inch rather than 6 inches. Then doing the same sensitivity study on flow, if flow was 1000gph instead of 500gph, delta h would be 1.5 inches. etc.

Another consideration: if you wanted to stick with 1 inch holes only, you could simply add more holes to achieve the same effect on delta h as a single 2 inch hole. But remember when we went from a 1 inch hole to a 2 inch hole, the hole area got 4x larger. So intuitively, you'd need four 1 inch holes to have the same effect as a single 2 inch hole.

If you're comfortable with doing the math, you can use the equations I gave you to size your intake holes. if you know what you want to limit delta h to, you can use the 2nd equation and solve for a instead, and then figure out what size and number of holes gives you that a. As an optimization problem, the goal may be to minimize delta h. since all you really have control over is size and number of intake holes, minimizing delta h means maximizing a. This circles back to the simple answer I gave at the top: make the holes as big as possible.

If you don't want to do the math yourself, just post your filter flow rate and how far you'd be ok with the water dropping in the intake chamber behind the hole and I can run the numbers.
 

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Discussion Starter · #3 ·
WOW!!! Thank you so much for taking the time to write this up. I am using an FX4 on one side and an FX6 on the other side. So right around 1000gph flow rate between the two. I guess I hadn't even thought about it but I am sort of creating overflow chambers out of styrofoam. :-| The pic I included is what I am using for inspiration. I will be making a big section on the left side and a big section on the right side. Carving our a rectangular box in the back of each of a heater, intake and return. The hole will be drilled through the top for the PVC return to come through. Then drill a hole in the bottom somewhere to feed water back into the box for the intake to suck up. Does that all make sense? :) I hope.
 

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No problem, I'm an engineer so I find these sorts of calculations fun :). I think I'm following what you're saying.

I've never used canisters or FX filters in particular, so maybe you can clarify something for me. When I look at the specs on fluval's website, I see there is both a filter circulation value, and a pump output value.

FX4 filter circulation 450 gph, pump output 700gph
FX6 filter circulation 563 gph, pump output 925gph

So I see where you get the ~1000 gph total from (filter circulation) but how does the pump output value come into play? Do these filters have some amount of internal bypass? If so, I'm thinking you're going to need to use the pump output numbers to size your intakes.

Or is the pump output what you would get if you ran the pump only without any filter media and the filter circulation the number you get when you add the restriction of filter media?

Using the smaller filter circulation numbers, the side feeding the FX4 would need to have a 4-7/8" lower water level than the main tank, and the side feeding the FX6 would need to have a 7-5/8" lower water level. This is assuming the 1 inch holes are the only way water gets in to each corner chamber. You're exactly right about them basically being overflow chambers.

So three considerations. 1) You'd have to make sure your filter intake is at least that low so you don't suck any air. 2) you'd have to make sure your heater(s) is fully submerged enough to not become unsubmerged when the filters are running. 3) You'd have to be cognizant of the size of your styrofoam rectangular chambers because whatever volume of water that gets pumped out to lower the water level will have to be held in the main tank and will slightly raise the water level (~5gal per 1 inch water level in a 125). Probably not a problem unless your corner chambers are very large and/or you keep your tank filled up right to the brim, but just something to think about. Also some people like to keep their heater controls high and dry even if they're rated to be fully submerged, this wouldn't be possible for you with 1 inch holes.

I'd still recommend going with larger holes if your background can accommodate them. With 2" holes, neither side will have to drop more than 1/2" in water level compared to the main tank. And if you have to be capable of the full rated pump output 1" holes will probably be totally insufficient (11-3/4 and 20-1/2 inch drops with 1" holes vs. 3/4 and 1-1/4 inch drops with 2" holes).
 

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Discussion Starter · #5 ·
I believe you are correct. My best guess is the filter circulation (smaller number) would be with the media in the filter and up and running.

The styrofoam rectangular chambers are going to be about 3 inches deep by 6 inches wide by 21 inches tall. I can go up to 3 to 4 inches on the holes. The reason I originally asked about 1 in holes was that I ordered some intake strainers from Marine Depot and wasn't paying attention. I ordered 1 n diameter. I can get 2 inches from them or I can get 3 or 4 inch PVC drain intake covers and cover that with fiberglass mesh if need be.

So I can go bigger. I just wanted to gauge how big. Based on this, would 2 inches be big enough or should I jump up to 3 inches?
 

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Based on the numbers I ran if it were me, 2in would be the smallest I would go but I'd probably want to go larger if I could for safety (ie if the strainer got partially blocked). But really just depends on whether you want the water level in your styrofoam chambers to stay even with the tank or if it's ok if it drops some. If you want it to stay even, go as big as you can.
 

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Discussion Starter · #7 ·
Thank you so much. This has been extremely helpful on a topic I hadn't even thought about having an impact on things.
 

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You're welcome, happy to have helped!

JimSparks said:
Off topic...what is the fish in your profile pic? :)
He's a male Metriaclima sp dolphin, sometimes called Giant Demasoni, I kept about 10 years ago.
 
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