You can get a fair bit of gold just from fishing at Bassken lake & selling the goldfish & guppies you catch, as they're used for Alchemy. You're looking at a minimum of 300 gold per fish of these types in the MP, and maybe a bit more depending upon the type & how many are available at the time.
Other than that, the older large fish from any lake are reasonably lucrative, because they can be turned into fishing rares (1k large fish of 1 colour makes a particular type of rare).
There's other ways which can make you gold faster, but with the popularity of Alchemy, fishing certainly isn't the low gold earning game it used to be. It's a pretty solid earner which isn't subject to any gold caps. I would say that a good fisher could probably earn 20-40k an hour at current MP prices, but don't expect to be that efficient if just starting out.
At the moment guppies and bubble-eyes are worth several hundred gold. Its a good way to earn if you sell the small fish. A bucket costs 25g for F grade. 100g for D grade 250g for A grade (unless you find them cheaper on the marketplace) At 18 fish per bucket goldfish or guppies would make your bucket work at least 5400g on today's market (using the cheapest price of the 6 fish used in alchemy)
The guppies and goldfish definitely are worth a good bit of gold, especially in bigger numbers. Bass can be traded up to green, gray, and blue stripers in the shop depending on how many you have (I don't always recommend trading in fish at the shop, but I make an exception for trading basses for cool color stripers) Stripers can also make for a decent profit.