Im going to go a little against the grain here.
Macro lenses take some practice due to the distances, light requirements and DoF. You might struggle initially and become somewhat disappointed. If you want to go through a lot of coin in photography fast, lens purchases and upgrades are a good way to do it. You also want to think about what kind of shooting you want to do outside of tank images.
One good lens everyone will want to have in their kit at some point is a fast lens. They come in so handy (family portraits, indoor sports, so many, many applications). Having a fast lens will also pay large dividends while shooting your tank. While a macro lens can be used for other things one thing all macro lenses have in common is brutally slow focus speed. Not a big deal if your trying to shoot a portrait of your child, but useless trying to shoot the same child say playing Volley ball in an indoor gym.
So a lens like a 50 f1.4 or f1.8 or an 85mm f1.8.
No you won't get the close ups initially but you can always add a set of keno Tubes which are pretty inexpensive.
When you add tubes theres no light loss and the tubes significantly reduce the lenses MFD.
a 25mm tube or stacked with another 10mm tube, mounted in front of a 50mm or 85mm will get you in nice and tight until such time as you look at adding a Macro lens to your kit.
Plus once you do acquire a macro you can always add the tubes to it and further reduce your MFD if desired and increase the magnification from 1:1 to closer to 2:1
85mm f1.2 with 10mm in tubes to provide some reference of scale
180mm f3.5 Macro and 66mm in tubes
same 180 and tubes
Again, to me anyways think first about building your kit. You will want to shoot things other than your tank and a fast prime will be more useful to you overall, than a 1:1 macro initially and by adding a set of tubes you can do some close up photography without needing a 1:1 macro.
J