Fresh2o
Aquarium Advice Addict
Now it appears another difficulty: my fluval is 17" sth then should i pick the 16 or 20" finnex?
Good question. Hopefully others who have done this before will chime in.
Until then, it may depend on if you have a lid on the Fluval. If so, then either should be fine. If not, then go with the 20".
If you go with the 20", you might wonder:
1. How the over hang (1.5" per side) will look?
2. How much light will be cast outside of the tank?
3. Can you use the leg mounts?
Answers....
1. Here is a front view of a 30" FugeRay resting on top of a standard 20 gallon long. It overhangs 1.5". I left the leg mounts in place. Please note that it is raised approximately 3/4" because of custom "rails" I added to the standard tank rim. If this does not look too bad, then go with the 20". If you ever need to use the light on another tank, then I believe it should fit onto a standard 10 gallon tank.
2. Here is an undershot of the over hanging fixture. I did not turn on the lights (too early) but you can see that no LEDs are visible (image might be a little dark).
3. The leg mounts will slide laterally on the fixture about 1/2-3/4" to allow some variance but not enough to mount on each side of a 17" from what I can tell.
Hope this helps and good luck with the build.