I was just thinking about what level of flow you actually need through an external filter. I know you need flow to distribute nutrients but this could be achieved with a powerhead. This is purely out if interest as I was wondering why more people don't just do something like use a small powerhead to drive a spray bar and drop to a less powerful filter?
Because strapping a rocket nozzle to the back of the tank is ugly. The idea of a planted tank is to create the illusion of a natural stream or pond and to remove, as much as possible, all visual cues of hardware from the tank which ruin the illusion.
Naturally, the first priority, as you mentioned, is to ensure the health of the plants, so if the filter being used is weak then the flow and distribution need to be augmented. It's a lot cheaper to buy a powerhead than it is to invest in a replacement filter with a more powerful throughput however.
My eheim 2217 has a capacity of 6l and I know that nitrification needs oxygen so if flow drops it could get anaerobic which would be bad but if you treat flow around the tank as a separate issue then I am guessing you don't need 1,000 lph to achieve filtration?
You might be missing the point here (or I may be misunderstanding your point) but you do not need 6L of filter media in a planted tank. The plants do the work and they bypass nitrification. In so doing they oxygenate the water better than any filter could dream of doing. Of course this assume the plants are healthy and that there is sufficient plant mass. Adding 6L of filter media will actually choke the flow and in so doing deprive the plants of receiving the nutrients and CO2 necessary for them to perform efficient oxygenation. From an aesthetic viewpoint, many people make the mistake of adding a powerhead to improve flow/distribution when they might have been able to make the improvement by removing all that filter media, especially those absurd noodles and ceramic balls.
In a fish only tank the element of the plants is missing so oxygenation has to be achieved via air bubbles and so forth.
In both cases however, frequent and large water changes help to remove organic material which will reduce the removal of oxygen by bacteria. CO2 fed plants produce a LOT of organic waste and so tank cleaning is imperative.
Assuming that the water is reasonably well oxygenated then would a lower flow rate actually cause issues? I am assuming the driving factor would be turnover so it would depend on tank and filter size but what rate of turnover is necessary?
But this is a big assumption that a tank is well oxygenated. Just exactly how is this supposed to happen. In aquariums, the commodity that is always in the least supply is oxygen. As I indicated above, the way that you
achieve "well oxygenated" water is to have sufficient flow/distribution in order to deliver the necessary raw materials to the plants for them to oxygenate the water.
Sometimes I think people "talk up" flow, is it really that critical?
Yes, I'm one of those who talk up flow all the time, and that's because I know and see that improved flow fixes a LOT more problems than does low flow. Sometimes, in a tank that is suffering poor CO2 or poor nutrition, the hobbyist can resolve the issues by simply removing filter media, or by simply rearranging the filter outlets.
Again, there are quite a few variables, such as plant variety, tank size, lighting and so forth as you noted, but every tank and every hobbyists is different and in order to solve some basic problems, attention needs to be paid to flow/distribution. So the 10X rule is just a basic guideline to help keep folks out of trouble. Does this mean that there are no exceptions or that variations are not possible? No but compare that advice to the traditional advice offered by the "add more light" camp, and you'll see that many more problems are fixed by attention to flow than attention to increased light.
I've noticed that a lot of advice being given by experienced hobbyist to beginners never take into account that the beginner really has no idea why a concept works or does not work, or how a deviation from a standard procedure can be used successfully. The person offering the advice subconsciously assumes that the beginner knows what he know or understands what he understands. I always assume the opposite because I know that many people are coming from a fish-only culture infected with all kinds of fears and misconceptions, so I always try to guide them towards the rules of thumb that we have seen in our experience that avoid common trouble. As the person gains experience we can see that they easily deviate from the basic advice to experiment or for convenience, but they can do so successfully because of being armed with a better understanding of cause and effect.
A lot of powerheads connect to pipework so I was thinking you could connect the pipework direct to a spray bar.
If you're going to use a powerhead then just use it the way it is and get on with it. As long as it's mounted properly, i.e. central and just below or just above the spraybar pointing in the same direction then it will be fine. Attempting to connect it to other components usually results in loss of efficiency or reduction of flow rate.
Cheers,
