Water flow and fish behaviour

[boyohboy]

Hi, I have a 20G tank with a Fluval 204 (up to 40G) canister filter. I have 7 Rummynose-tetra for about 3 weeks. They look great but unfortunately they always swim only at the very bottom (1-2") AND hide among the stem plants. I didn't figure out why and they look very health.

But the last 2 times I cleaned my filter, I noticed that while the filter was turned off, the Rummynose-tetras would start swimming out and up. I've reproduced the "effect" by unplugging and pluging the filter and so I'm pretty sure the tetras' behavior is due to the water flow.

I've just DIYed a spray-bar to replace the nozzle hoping to reduce the water flow speed. The speed is slower now but the tetras are still not coming out. :shake:

Any help will be appreciated.

PS. How slow of a water flow is acceptable to at least provide enough water circulation for filtration purpose?

[beviking]

That's a tricky question.
Biological filtration (breakdown of ammonia and nitrite by bacteria) is more efficient at slower flows where mechanical filtration (the actual removal of particles floating around) is more efficient at higher flows. If your flow is slower, more particles will settle into the gravel. As long as you vaccum the gravel this shouldn't pose a problem.
The 204 is rated at 180gph? For 20gal tank, thats a 9X turnover rate - meaning the total volume of water in the tank is getting filtered 9 times per hour. That's a little high IMO. Many ppl will tell you to shoot for at least 10X per hour though. I had a 55g with a turnover rate of 12X/hr and my tetras did the same thing yours are doing. Personally, I would try to slow down the flow (go for 6X per hour) since you want your fish to swim around and look somewhat natural. Unfortunately, I don't have any advice on how to do this w/o getting a different filter.

Good luck!

[boyohboy]

Originally posted by beviking
I don't have any advice on how to do this w/o getting a different filter.

Well the flow rate of the Fluval is adjustable, so it's not so bad ;)
But it's a bit of wasting filtration power so to speak, I duno.
Last nite I've just re-made the DIY spray-bar again, drilling many more holes but much smaller holes. The water current seems to be again abit slower. As of this monring, the rummynose actually did swim out somewhat in the clear (altho still close to the bottom/gravel), hope they'll get even better later, finger crossed..

[beviking]

Oh yeah, the spraybar. If you direct some of the current in different directions, you won't have the whirlpool effect (all the current going in one direction) and they should handle that alot better. Does your spraybar have sections? Just point them diferently or since you're into drilling holes, drill a few to aim water straight down, a couple diagonally. I actually did this in my tank b/c the current was laying the plants flat on the other end of the tank!:eek: