Hey! I liked this game's concept and execution (particularly the fish art and the music selection), but I had a couple of pieces of feedback. Feel totally free to disregard them, but I figured I'd throw them your way in case you found it useful. I think the core gameplay loop is tested and true - it's basically gambling or playing the stock market, giving you dopamine when you make a good sell and a sense of 'I'll do better next time' when you sell poorly.
I think labeling the 'menu' option as 'Main Menu' would be a nice change! I clicked on it out of curiosity on my first run, expecting options or additional info, but it reset my progress instead. Maybe a dumb move on my part, but I think that change might be a nice idiot-proof measure.
I think something I struggled with a bit was having the sense of appropriate fish value too obfuscated. Some obfuscation is necessary, otherwise the game can quickly stop being a 'game' and more of a 'if-this-then-that' flow chart. A simple change might be to display an up or down arrow next to each fish based on if the fish gained or lost value so players can readily tell what decisions they made on the previous day were good or bad.
Totally spitballing here, and maybe it may not work in practice, but something I think might help is considering giving each fish a 'weight' or 'momentum' based on the previous few prices of a fish. If the fish has been fluctuating in no particular direction, the new price can be totally random. But if the price of the fish has been consistently increasing over the past few days, maybe there should be a proportionately higher chance of the price continuing to go up, but also a chance of that price absolutely plummeting very low. The inverse could also be true; consistently decreasing values can either continue to tank or shoot up drastically in value. This could be a good risk-reward system that allows players to make more interesting decisions.
I have no idea how much time you intend changing or adding to this game, but those were a few of my thoughts, off the cuff. I want to reiterate that I liked what was here and had a good time playing it! Hopefully this feedback wasn't unwarranted or too critical. :)