I have not shot with either of those two bodies (I have a d80 that I love
lol), but in doing a quick read on the two, it seems like there's some improvements on the d5300 that will make a difference in shooting little fishies, assuming this is of importance to your buying decision.
I am primarily looking at the d5300 shooting speed and light sensitivity on this point. The greater ability in those two areas will make it easier not only to capture macro shots of little aquatic critters that suddenly and unexpectedly move, but also easier to capture an overall shot with a school of fish swimming by at just the right moment. Also, in the grand scheme of dSLR cameras, a difference of $100 isn't really that much, considering the 5300 body will likely give you advantages in some key areas assuming your focus is on this type of shooting.
Now that being said, your glass cannot be ignored either. Good lenses also make a world a difference, and are typically much more of an investment than the body. On the up side, once you buy a lens, they typically carry forward to newer models, since Nikon tends not change its mount. For that matter, very old lenses from the non digital days can often still mount and function on today's cameras, you just do not have certain features like auto focus, or light metering. Please do check on a lens before actually trying an old one on today's cameras, though. There are a couple of the old ones that are not suitable and can damage a newer body.
The final suggestion I would give you is to try both bodies out and see what you think. You may even check and see if there's a place that does camera rentals in your area, so you can take both bodies home, and get a real feel for them. At the end of the day, the camera needs to please you, and not anyone else.
Happy shopping!