Also took fish pictures with my Pentax DA40mm Limited.
From my experience, taking pictures with the kit 18-55 or the 55-200 just wouldn't cut it. Fish tanks are normally very low lighted, and the white balance is very very difficult to get properly (different fish tank units have different lighting setups... ranging from daylight white to bedroom white... sometimes both in one ballast!).

The Pentax DA40mm Limited's lower F-stop of F2.8, and a bit of experimenting with white balance (and some composition experimentation), allowed me to FINALLY take some decent pictures of my fish! (those fast moving fish are SO hard to take!)

[Pooky125]

What a pretty planted tank. Specs?

[dugrant153]

The tank is lighted with a 110W daylight halogen bulb ballast, but only in the front (this creates shadows in the very very front, and in the back. I think the fish like this as it gives them a place to hide). The tank consists of about 40 fish or so, and all the plants are live.

I used my Pentax DA40mm Limited, ISO1600.

[lapeen]

Gorgeous planted tank. I tried started on up again this year (Years ago I had a gorgeous, thriving one with Co2 and everything)... but I just don't have the patience and desire to put all the work into it, so most of my plants are currently in the process of dying.

I think if you messed with the WB sliders a bit in PP you could get the photos looking more natural... a little less yellow from the lights.