A 50mm 1:1.7 is a 50mm f/1.7. They mean the same thing. That will be perfect for full tank shots, assuming you can stand several feet back.
As for macro lenses, doing just a quick search on B&H, I thought these would work well:
- not super cheap, and I'm not terribly familiar with the brand since they've merged, but I know Konica used to be a really nice company, and Minolta too, so this might be just what your looking for.
- This would be awesome. I personally lust after one for flower and bug shots, but you might find it a bit much for fish shots.
- this is a nice budget lens. I know you already have a 70-210, but this one, apart from having a slightly longer reach does a nice closeup up to 1:2. Not quite as close as the other two, but nothing to shake a stick at.
- Another macro, cheaper then the other 2, Sigma's make nice lenses. I have a Sigma 90mm macro for my DSLR and I love it.
- I add this one, because I think it needs to be said, that I don't know anything about it, but pretty much, you can't go wrong with any macro lens. Any of the 3rd party companies make a nice one and I'm sure Minolta does, and I wouldn't be surprised if Sony had a nice one too.
- I know there's someone on this board who uses this with a Canon DSLR and gets some nice images, I wouldn't rule this one out either.
- this one, is different then the rest. Instead of being in the 90-180 range, this is a 50mm. Could kill two birds with one stone. It's a macro lens, and would be good for a full tank shot. Not to mention, it's not terribly expensive to boot.
So, to sum up, pretty much any macro lens you choose will be just fine. There are several suggestions depending on how much your looking to spend. If your going for a budget true macro, I'd say the Sigma 105mm myself. You won't have to invade personal space, and spook your fish, but it will still give you that nice 1:1 magnification.
The 50mm f/1.7 is nice, and it would suit you well for low light shots, not just of your tank, but any family gathering inside, but you could always kill two birds with one stone, and get the Sony 50mm macro. You won't find it quite as fast for those low light shots, but it should suit your purposes just fine.
HTH, and I didn't fill your brain with to much at one time.