I know our local sculpins are very high oxygen demand fish and are found in very fast/cold water. From my research keeping one of them would require a chiller. I'm not sure about the variety you have locally, but it would be worth some research, as they may not do well in warmer waters.
Sculpin are generally a colder water fish. However, I'm pretty sure I've seen them in the section of the James River I'm going to be mimicking. However, I could have just seen some kind of mottled darter and assumed it was a sculpin. The water definitely gets warm there, we wade/swim there all the time.
If they aren't there, they wont get caught and put in my tank!
I am so jealous of you getting this tank for free, it's incredible! This is exactly the sort of project I wish I could be doing. Aside from getting the free tank, do you have an estimate on how much this project is going to set you back? Itemised if possible? (If you don't want to say, that's cool)
Yes, that system is almost exactly what I was imagining. The only difference is that I will be adding a "marsh" area for some filtration instead of just the sponges.
I am starting to think about light now. I wanted to put this in an east facing window, but my wife wasn't too excited about that. Looks like I'll need to build some lights!
Again, I'm thinking super low budget here. I'll probably do 5 cfl bulbs in a custom reflector. Possibly two timers so that I can simulate a day cycle. Not totally sure though.
I would not recommend extending the intake. Instead, put the powerhead itself under your sponge in the compartment at the far right end in your diagram and use vinyl hose or PVC pipe on the output side to get the water to the other end of the tank. Besides being better for the pump (small centrifugal pumps really don't do well with restricted intakes) it means you can hide the ugly thing under the sponge where no one will see it, instead of having it hanging in plain site in the display portion of the tank.
I know it seems like that's the case logically but from the pump's perspective it's definitely not true. Restricting the intake causes pressure inside the pump to drop, which can lead to cavitation and really poor performance. The performance drop might not be an issue for you but the pump will probably not last as long.
If you use a larger diameter pipe for an extension, it should make up for the restriction you might have to go a few sizes up. You can also do something like this it will pull the water through the filter and make flow across the tank but the flow will bounce back eventually and create more of a whirlpool. I'm not sure if that's what you want.