I posted on this in your other thread where you asked about the lighting, but I will copy that over so we are all on the same page. But first, on something mentioned here and so you understand, there is no problem in having two different types of tubes; it is the types that matter. In your case, I suggested replacing the 48-inch Aqua-Glo with a "daylight" 6500K tube because the AG is half the intensity of the same sized tube in 6500K, and that makes quite a difference. So the 4-foot 6500K plus the 3-foot 6500K will be good light. I use combo lights on my dual-tube tanks, or have; but the tubes themselves have to be good ones. This is how we can achieve a warmer or cooler hue in the tank, by using one 6500K tube plus one warm (lower K number) or cool (higher K) tube with it. Provided they are all good tubes.
Now I'll just copy over verbatim my comments from the other thread, some of which will duplicate what Geo and others have mentioned. Presumably there is a glass cover over the tank, so you are free to select any fixture.
This is a 4-foot tank, so first off, I would go for a fixture that takes 48-inch tubes. This provides full coverage end to end, but also the 4-foot tubes are more readily available in the less expensive brands. I for instance cannot find these in some sizes, but every store has 4-foot tubes. Long-term considering replacement, this can be a significant difference in money.
Second, if you can find a single-tube T5 fixture, that is a good option, But not a dual-tube T5, that is way too much light; I tried one for a week and took it back. I can't get single-tube T5's, but another member in Texas I believe mentioned he could, so I throw this out.
Depending upon the above, the next best would be a dual-tube T8 fixture. You will have to watch the duration, as this is a bit more light; too long and algae will become an issue.
As for tubes, i already covered this. Those with a Kelvin around 6500K, regardless of T5/T8. T5 tubes are more expensive than T8. And whichever, they have to be replaced regularly; T8 every 12 months or so. Some sources say T5 last much longer but I've never used these permanently so can't comment.
Last edited by Byron; 10-20-2012 at 02:01 PM..