Originally Posted by WisFish
I've heard the same thing about surface disturbance. But I think it's a catch 22. On the one hand you want the surface disturbance to add oxygen to the water for the fish. But you don't want it disturbed so the CO2 from the fish stays in the water for the plants. But at night the plants don't use CO2 but instead need oxygen. That's why some people use air stones at night.
I use an Ehiem 2217 on my 55gal. The instructions say to aim it at the surface to cause that surface disturbance we're trying to avoid. But if you don't aim it at the surface, you get such a surface protein scum that food won't even penetrate the surface.
So I still use a UGF with an air stone to breakup the surface protein and get some oxygen into the water for the fish. I'm guessing because I use Excel for a CO2 equivalent source the surface disturbance isn't as much of an issue.
But the original question I thought was do you need to replace the HOB filters to have plants? I say no. Are other filters more desirable? I'll let the other make the call. But it's not necessary.
If I may, a couple of comments WisFish. On the oxygen, in a planted aquarium (and we're talking planted throughout, just so it's clear) the plants produce far more oxygen than the fish can possibly use unless you are way overstocked. Another fact is that high oxygen levels make it difficult for plants to assimilate nutrients. Peter Hiscock comments that "it is best to use filters with low flow rates, or undersized external filters to reduce surface movement and keep oxygen levels low. Lowering the outlet of a filter will also help reduce surface movement in the aquarium."
One must remember that we are dealing with natural or low-tech systems here, and the balance between the 17 nutrients (one of which is carbon) and light has to be there; so anything that may impact however slightly can become a critical factor in less success. And there is definite loss of CO2 through surface disturbance, airstones, powerheads, whatever. During the daylight, this should be avoided. The one thing we cannot "control" in this type of setup is the CO2, by which I mean that it is entirely dependant upon the fish and biological processes; with light we can control it, in intensity and duration, to balance, as we can with the other macro- and micro-nutrients through fertilization. Plants will photosynthesize up to the factor in least supply. Many have planted tanks fail because the CO2 is the limiting factor, and algae will take over because it is better able to use carbonates for carbon than most (but not all) plants. Won't go into all this here.
Submerged plants have difficulty obtaining enough CO2 in nature and in the aquarium; this fact is believed by many to be the reason for the inherently slow growth and low productivity of aquatic plants over terrestrial. Further, freshwater emerged plants have been shown to be more than four times more productive that submerged plants. The reason is because CO2 diffuses so slowly in water as opposed to air, and this limits the underwater plant's uptake of CO2 because the CO2 molecules don't contact the leaves quick enough to meet the plant's needs. Aquatic plants have to use enzymes to rapidly capture the CO2. When the CO2 levels in the water become depleted, these enzymes sit idle, so to speak, but the plant still has to provide energy to them. This results in a reduction in photosynthetic efficiency and therefore growth of the plant because energy is being wasted. Thus, any thing that removes CO2 in however small an amount will be detrimental to the plant's growth.
Increasing air/water movement at night is obviously quite a different thing, and in a high-tech aquarium often recommended. It is not necessary in natural systems because the balance does not warrant it. The plants produce oxygen in excess of what is required, and without surface disturbance it will stay in the water and the fish will use it at night. Your Eheim folks are thinking of the average aquarium with few if any plants, and oxygen/CO2 exchange is far more serious because there is no natural process there to deal with it. Just another example of how in a planted aquarium nature can handle most things better than we can, and she should be allowed to get on with it without our meddling which in the end is probably going to be detrimental.
On the surface protein scum, yes; I never get this in my 90g, I have it very minimally at one end of my 115g, and it can get quite heavy in my 70g. But I know the balance in the 70 is out of whack; I've dumped plants in it to get it going, and have not yet aquascaped properly because I'm waiting for some particular plants. I used to use surface skimmers that you can buy for Eheim canister filters; worked fine, the "scum" never appeared because the water very gently kept moving but not sufficient to be even seen. I removed these when fish kept getting caught in them.