Hey qinapses, it's been lovely watching you develop this game. I just played it and since the only thing you ask for is feedback to help you improve, here are some of my thoughts in terms of art.
First, things I already quite like: The fish animation is simple, but solid and serves the purpose well. The general palette of the game and feeling it gives is great. Lovely cyan background, nicely countered with the goldfish that stands out a lot, like it should. The enemy fish also communicate nicely that they're dangerous. At first I was a bit annoyed with different resolution of the background and the fish, but it does serve the purpose of differentiating the two even more and in the end I'm not really bothered by it.
Things to improve: The foreground floor is moving slower than the fish, which makes it seem like it's in the background. Yet, the grass is layered in front of the fish. The only physical explanation for such movement is that the camera and the goldfish are very slowly moving to the right, while all the red fish are blazing in to the left. If your intention was to make the red fish stationary and goldfish try to run past them, then either you have to push grass in the background (so it moves slower), or you have to speed it up, faster than the red fish.
I'm not sure if you used any references drawing the floor, it seems a bit random. The bigger problem perhaps is very 'gooey' shading (like it's a translucent gelatin). Things underwater have a quite prominent hemispheric lighting setup with strong shadows underneath. Backlit setup would work here nicely as well I think, giving the foreground a nice black silhouette in front of the playfield. You could have multiple layers of grass with progressively lighter shades to give the scene more depth. Some layers could be grass and waving slowly around, but that's asking a lot. :)
The texture on the background works a bit against you I think, because it falls very flat. The layers of increasing lightness work well to indicate depth, but since they don't move in parallax, they give the impression this is painted on some cardboard layer in the background. A linear gradient (not necessarily a smooth one, it could be in 5–10 palette steps) would probably do a better job. You could add a few layers of god rays, slowly pulsating in intensity and overlapping to give more interest to the background (but very slight opacity so it doesn't become distracting to the foreground).
All I've said creates quite a bit different art direction, so take that into account—it would make the game look quite different, which might not be what you want. It's more of an alternative look that has more depth/volume. It might also lose the clear distinction of foreground/background, so you'd have to be careful and probably adjust some things to go with it.
But in general, learning more and improving shading+lighting setup is probably what would give you the biggest addition to your skillset at this point. Especially confidence with your shadows. Ask yourself, how would you shade things if you had only black and white? right now you're relying on slight changes in value to indicate some sort of shading, but not really commit to it. Black and white will force you to decide what is in shadow and what is not and think about it from a geometrical standpoint.
Alright, I hope this is helpful. I'm not going to comment on gameplay since it's a clone, but form a technical standpoint it's very well executed so props on that! Finishing any game is hard. Since you code so well and also improve at art all the time, you're up for great things in the future (not just flappy bird clones :))! Excited to see what you bring us next.