@BJcorals I'm a photographer by trade and I have a degree in fine art photography and 18 years of experience.
The best money that can be spent is in your lens.
Frankly any camera would suffice (image production) these days. I've shot with Canons and with Nikons. As far as one being better than the other it comes down to personal taste.
As for my opinion, if you were a friend of mine I would tell you to buy a Canon camera specifically. In my experience Nikon cameras tend to be a tiny bit better; Canon the lens tend to be a tiny bit better. Again you can't go wrong with either, that's just my opinion.
The ability to shoot in raw, will enable you to fix any errors made with lighting or white balance (ie educate yourself using Adobe Lightroom). Generally getting a pro body with a few generations ago gives you every major tool and feature at your disposal for less money.
In fact I would recommend not using camera lighting when working with macro images. The natural light is going to be far better in aquariums and any flash you add is going to simply add glare.
If you're looking for a good general camera go to KEH.com. They're the largest used camera dealer in the world.
I'd recommend getting a Canon 5D classic (or perhaps a Canon 5D mk2 if cash permits) (this is a pro body, and a full frame camera) and its <$300.
https://www.keh.com/shop/canon-eos-5d-12-8-megapixel-digital-slr-camera-body-only.html
Some of the things you should look into is a good quality macro lens. Bigger isn't always better with Macro lenses. Anything around 200mm has a longer barrel and you need to have stabilization to get crisp images. These lenses are designed to be used with focusing rails. I would recommend if you want something professional you need to look for a lens that has a built in image stabilization of some kind.
Keep in mind that for the most part you can hand hold focus a macro lens around 100mm and I would recommend getting one of the following. Below are the 100mm basic variations for Canon the top listed is a "L" class lens these generally are weather/ dust resistant which might come in handy shooing around aquariums. It is also a lot sharper than the other but, for the price you can't go wrong.
Top of the line @ about $650:
https://www.keh.com/shop/canon-ef-100mm-f-2-8l-is-usm-macro-lens.html
Much cheaper @ about $300:
https://www.keh.com/shop/canon-ef-100mm-f-2-8-macro-usm-lens.html
If you would like to try them out they can be rented @ Lenrentals.com or lensprotogo.com
Something to mention is often times shooting in water creates glare. "Some" macro lenses allow for a filter to be screwed on, I would recommend a good quality polarizing filter (don't skimp out on this, a poor quality filter in front of an expensive piece of glass is worthless).