I have a few tips for you.
1. The first thing is to go on youtube and look up at least 5 well done videos on the exposure triangle. That is the BASICS and you need to know the basics before you can get good photos.
2. You DON'T need photoshop or lightroom. Canon used to have a software that came with the cameras and was decent to start with, but it wasn't anything super nice or for anything more than messing with the exposure and stuff.
If you want a good look at the best software, then look up J Christina on youtube and watch his "Life after Adobe: cutting the cord" series. I don't have any affiliation with any party involved, but Adobe is expensive, and very invasive. There are other softwares that work just as well, and only cost ONCE, not once a month lol
3. You'll want new lenses eventually and the BEST way to get them is used. That camera (the Canon RP) is actually not a DSLR (or at least not a NORMAL one). It has no mirror, so it has a lens, and then the sensor, no mirror is involved in passing light up into the viewfinder or moving out of the way of the sensor like in DSLRs.
That said, it is a simpler and arguably more robust system that has a very nice advantage: a very short flange focal length. That basically means that you can (theoretically) get adapters for most lenses that don't need optics (Meaning they can be much cheaper). I have a Sony with one of the shortest FFLs, but I think the new Canon mount is even shorter. Anyway, you can get old lenses WAY cheaper than new ones. Canon and Nikon are probably the best two lenses to get used because they are very common, but you can get others too.
One of the first lenses you'll want to get is a macro (preferably with autofocus) for the fish. You can use others, but a Marco lets you get in close and really "zoom" in.
4. To get better photos of your fish, you'll want a larger depth of field. Larger f/stop numbers (smaller/slower aperture), and shorter lenses (shorter focal length) both give you that.
5. Don't make the same mistake as me. ISO is great, but the higher it is, the noisier (grainier/more pixelated) the image will be. I shoot with it set to 100 and only change it if I have to to get a shot. It isn't the end of the world, but (with the possible exception of DXO de noise software) no post processing is going to remove it without leaving the photo less sharp looking.
6. There is NO shame in shooting auto. The drawback is you WILL miss shots and you will not learn what everything does. Manual is better since you learn what everything does, and have more creative control, but P and S modes are good too (they let you set the shutter speed or the aperture and the rest is auto).
I shot in P and S modes for a few years and then transitioned to manual.
I hope this helps. I know it isn't fish specific, but it will definitely help