[niko]

Funny thing you ask about flow. We seldom discus flow in planted tanks but it has a profound effect on the plant's health.

To reiterate a few well known things:

1. All canister filters are rated wrong. If a filter says it moves 350 GPH that does not mean it actually does it 2-3 feet above the canister housing. Often the flow is cut 3 times. L shaped elbows, ingenious in-line reactors, and so on really reduce the flow.

2. Plants have a strange, magical, ability to block flow. VERY efficient. A spraybar placed on the bottom of a bare tank may move the entire body of water in a circular manner. But when the plants are in only a few inches of water in front of the spraybar actually move.

3. Differences in temperature due to uneven flow are very common. Even in a 55 gal. planted tank with a Fluval 404 (300 or so gph filter). Nutrients and waste products are not distributed/dissipated properly.

4. In the course of a week the filter manages to get clogged at least a little bit. Often the flow is visibly reduced, but usually it appears fine. Either way - the flow on Day 1 is not the same as the flow on Day 4. Is that a bid deal though? If you think that the plants adapt and it's all fine and dandy because you rinse your filter material every so often then good luck. Very few filter materials are engineered properly - to not clog easily. Most will "help" you and get clogged pretty fast. They will clean the water faster because of the progressive reduction of the orifices through which the water has to flow. You end up with a pretty clean water, reduced flow, reduced biofiltration and all that good stuff. Rinsing the sponges in aquarium water seems like a good idea. But it's not - disturbing the organisms that form the biofilter is the most persistent of the many idiotic habits we have in this hobby.

So after all of the above back to the original question. The most flexible way to add flow to a tank (big or not) is with small powerheads. Place them wherever you find they work best. If the tank is heavily planted you can hide them. Move them as you find appropriate if you find that the water movement is not good enough. Don't think that if flow is needed in a certain direction pointing the outlfow that way will always work. You may find that the best powerhead positioning is not very intuitive. And don't forget to remove the sponges in the small powerheads (see #4 above).

Or you can build an elaborate undergravel piping and hope you got all bases covered.

--Nikolay