Flow Rate Dilemma

[emieczko]

Let me preface this with an admission to being a newb. I'm an open minded newb, though, so I know that nothing I state is "fact," and I take criticism very well .

I currently have a 20g freshwater tank. I'm planning to spend the next year learning as much as I can with the goal of moving to a reef tank in the neighborhood of 100g.

The struggle I'm having at the moment is setting up a good flow rate for a scrubber in my current tank. The recommendations seem to be for shorter/wider screens and that's driving fairly high flow rates. My tank is populated by mostly very small fish, 1" length range. I don't know how they'll react to turning over the tank volume 8-12 times an hour. Can some of you more experienced folks chime in?

Is it an option to run a lower flow rate to/from a sump and then run the higher rate within the sump circuit back through the scrubber? In other words, I could put 2 pumps in a sump. One supplies the overflow on my tank at a rate, say, of 80gph. The 2nd pump supplies the scrubber at 200-250gph and the scrubber drains back to the sump. I know that would get the flow rates where I want them, but would that provide adequate nutrition for the screen?

Eric

[inkidu]

One time I thought I new everything then I woke up.

Welcome to the forum.

I believe that width to height ratio has more to do with optimizing your scrubber.

Don't let that rule over other possibly more important considerations.

If you want the correct flow along a long/short screen or a short/tall screen

have a way to adjust the flow. Some of the people here use a ball valve to adjust flow.

Hope this helps.

[SantaMonica]

Keep this one simple: Just use the overflow that you have now:

Screen Width-----Gallons Per Hour (GPH)

1" 35
2" 70
3" 105
4" 140
5" 175
6" 210
7" 245
8" 280
9" 315
10" 350
11" 385
12" 420
13" 455
14" 490
15" 525
16" 560
17" 595
18" 630
19" 665
20" 700


If you are doing an overflow feed, the overflow will determine how much flow you have to work with. You have to start from there, and size your screen accordingly. The maximum flow you'll get to the screen will be what's going through your overflow now. This is easy to figure out by counting how many seconds it takes your overflow to fill a one-gallon jug:

60 seconds = 60 gph
30 seconds = 120 gph
15 seconds = 240 gph
10 seconds = 360 gph
8 seconds = 450 gph
5 seconds = 720 gph


Take this gph number that you end up with, and divide by 35, to get the number of inches wide the screen should be. For example, if your overflow was 240 gph, then divide this by 35 to get 6.8 (or just say 7) inches. So your screen should be 7 inches wide.

How tall should the screen it be? That is determined by how much screen area you need, which is determined by how many gallons you have. Try to get one square inch of screen (lit both sides) for every gallon. If lit on only one side, double the screen area.

When finished, this is how you want your flow to look:

[emieczko]

I see a lot of conflicting information on the net (no surprise) regarding what flow rate one should target, irrespective of filter type, in the tank. Most frequently, though, I'm seeing 4-6 times/hour recommended for FW tanks with small fish.

If that's the case, I'm looking at flow rates in the 80-120gph range. At 35gph per 1" of screen, that puts me at 2 1/4" - 3 1/2" screen width and 7" - 10" of screen height. Since most of the scrubber designs I see highlighted here, certainly the Santa Monica designs, use L:W ratios directly opposite that, I "assumed" I wanted to head that direction, lol. Perhaps the problem was in my assumptions...

Eric

[SantaMonica]

Wider is better for high performance, because it processes more water. But your goal here is just to get some scrubbing going, cheaply.

[tanan]

I dont think flow rate for small tank has to be around 4-6 times/hr.
I had gone as much as 20x/hr with small fishes like tetras,baby rainbows,loaches,small angels,catfishes etc
And they wont mind the flow but its not like that flow was blowing them away,there were hindrance(rocks,driftwood,bog wood,plants etc).
You just dont want a flow so strong that it blows your fish away.
In planted FW tanks we usually go around atleast 17x/hr to avoid dead spots/algae in densely planted tanks.

[ocean rock]

dont run the ats from the over flow its almost impossible to set up have the screen run off a pump and have the over flow run into the sump trust me i know ive spent ages trying to set mine up and to be honest pumping it is the way to go .

[emieczko]

The note on higher flow rates is encouraging. I guess I'll have to experiment a bit and figure out what my fish will tolerate. Perhaps I can set up a pump and run 10x or better for a bit and see how they respond.

I'm a little disconcerted to hear about problems running the scrubber from the overflow, though. I don't really want to mount it above my tank. I'd like to put it below on a sump. I'll have to do some head scratching there.


Eric

[SantaMonica]

Over flow is certainly harder than a pump, but a lot of folks are doing it (probably 25%).