Macro for 200 notes is always going to be about compromise. You can get an entry level DSLR + kit lens, then use reversing rings, extensions or a Raynox adaptor. These will get you high level of magnification, but at the cost of fidlyness, light loss, extremely narrow depth of field and (perhaps impossibly) short working distances.
Or a very cheap older second hand DSLR and a legacy macro lens (Nikkor 105mm f4 macro, or similar), which should provide good images with no downsides other than needing to manual focus, not being much use for anything other than macro unless you add another lens down the line, and possibly crap high-ISO performance in low light from ending up with an old camera with an out of date sensor.
Or you could get a superzoom bridge camera of some sort and a Raynox or other macro adaptor. Again, light loss and working distance will be a challenge, but perhaps more useable in an aquarium setting and image quality will suffer somewhat compared to an SLR.
If it were me I would probably spend ~£250 second hand and buy one of the last-gen micro four thirds bodies (GX1 springs to mind) and an Olympus 12-50 lens, which has a very useable semi-macro mode. You won't get extreme closeups but it should work well for whole-shrimp shots, whilst being a damn site easier to use than the other options and also making a great all-round setup for general photography.