You are trying to cover lots of bases but I think I should be able to assist. I'm a freelance photographer, cigar smoker and aqua-hobbyist. Looks like we share the same passion.
Let's get the basics out of the way. The brand doesn't matter. Both Nikon and Canon are top notch companies and make extremely well designed equipment. Flip a coin and go with that company. Now, if you already have 'something' invested in one of the camps, let's hear about it. If you want to get into aquarium photography, you want to use a lens that has functional Auto-Focus. Those old lenses that you have will most likely work on your dSLR but auto-focus will probably not.
In order to extract every bit of detail to create good composition, I recommend that you go with macro lens. Depending on working distance (distance from lens to aquarium glass), you may want to buy 60mm or 100mm (Canon) / 105mm (Nikon) or even 180mm (Canon) / 200mm (Nikon). Also, all depends on your budget. You can buy used lenses for a really good price !
Based on your description, I would stay away from Pro-dSLRs and look into entry-level dSLRs. Again, all depends on your budget.
To see my cigar photography - check out -->
http://cigarphotography.net
Here are couple SUPER OLD
underwater aquarium shots: