Best fish to sell
Eight fish regularly sell for 100+ gold each: Four-Eyed Butterfly Fish, Spikefoot, Yellowtail Damsel, Emperor Angelfish, Tiger Puffer, Tuna, Betta, and Tilapia. Emperor Angelfish and Tiger Puffer are the rarest but pay the highest. A fishing net (unlocked at senior angler rank) lets you catch multiple fish at once.
Where to fish
Both lakes and oceans contain profitable fish. The south beach is reliable for Yellowtail Damsel and Spikefoot; the inland lake near your homestead is good for Tilapia and Betta. Tiger Puffer prefers the deeper ocean tiles — look for the larger shadow ripples.
Bait and spawn tricks
Bait is gifted often by NPCs and through bulletin-board rewards. Using bait forces a fish shadow to appear — so even in a fishless area, eight bait will spawn eight fish shadows. Combine bait with the fishing net to clear an entire patch in one cast.
Selling for max profit
The merchant boat at Star Sand Port pays a 200% markup on most fish — turning a 100-gold Emperor Angelfish into 200 gold. Unlock the port by talking to Marston on the beach; the boat visits roughly every three in-game days.