This is a cool picture with a cool thought process in making it. It's very funny to imagine.
I like the textures you've used in creating the water and the brushes used in the plants in the bottom of the tank. It looks very well done. I would to have liked some items or something in the background just to break up some of the simplicity of the water and grass. Even like a random fish/Pokemon in the background would be fine. Although I like the brushes it looks like you just put the same couple of brushes in different spots at different opacities. I would have liked to see a bit more diversity in that sense. Maybe by changing/ using a more diverse set of brushes it would have looked better.
The colours look good to me. The seagrass looks about the right colour, and the water looks good too. The shading looks fine on Gastrodon, but its not very well defined, maybe the water is cloudy.
Gastrodon looks pretty fine in this, no complaints.
In the end this is a pretty good piece, but it does have some little faults. I'd have to give you a 45 on this though, as the good outweighs the bad. Have fun with your little sea slug!