ive kept turtle far longer than ive kept fish... almost 20 years now, and the #1 rule with mixing turtles and fish is this: never ever get attached to any fish you add, because they can become food at any time.
the pleco i have now is about 6" or so, and he lived the first couple years with my (then hatchling) florida redbellied cooter (the pleco was about 1" when i got him) with no problems at all... when the pleco got too large for the 29gallon tank, he moved into the fish aquarium. i recently purchased another small pleco and tried to itro him to the same turtle with very bad results. even a pleco makes for good food aperently. i also kept an electric blue cichlid with that same turtle for a couple years and also never had a problem there. it just depends on your turtle and the fish. you can never say any fish is going to survive when there is a turtle present.
also, what kind of turtle do you have?
different turtles can be kept in different levels of water. if it is a mud/musk turtle its going to need far less water that your average aquatic turt. but if its a painted or slider or other fully aquatic, then you can fill the tank about 3/4 of the way. however, a 1=2" turtle shouldnt have more than a few inches of water untill it is bigger/stronger and able to better reach the surface.
i wouldnt use rocks for basking, as it can scrape the carapace and lead to shell rot or infection. i use large pieces of floating driftwood. rocks for underwater structure are fine as long as there is nowhere the turtle might get stuck and drown.
if you have a mud or musk turtle, or possibly a southern painted, a 10 gallon might do, but if you have a midland, western or eastern painted, or a slider, cooter or other larger aquatic turtle, you will MINIMALLY need a 55gallon setup within a year or two depending on your turtles growth rate.
as for turtle filtration, i use the Fluval 4Plus canister. one of those in each 55gallon turtle tank i have does a great job.
and yes, the
www.turtletimes.com forums are a very very good place to read up and get help with alot of things.
sorry for the long post,
~KM