Disney Dreamlight Valley

Your progress for this can be checked in the dreamlight tracker, in order to count towards the achievement it must be fish that are caught and not seaweed or seafood (crabs/lobsters etc).

The top left pond in the forgotten lands nearest the fast travel well appears to be the best place to do this as it has a single fishing node point that re-appears pretty much as soon as the previous one is used and being such a small body of water, has no travel time to get to the next node as you would have to in the other zones. It only saves a few seconds each time but that's a few seconds multiplied 1800 times.

I'd personally avoid just fishing in open water as although the fishing QTE is quicker (needing only 1 tap instead of 3-5) this will frequently result in catching seaweed, clogging up your inventory and not helping with the achievement.

George's guide is decent for finding the fishing pole, but doesn't really explain how to fish. I put together this alternate video that also tries to capture HOW to catch a fish in Dreamlight Valley.

To catch a fish, approach a pond and press to cast your line, preferably towards a bubbling circle in a pond to guarantee you hit a fish. This will start a small circle moving away from you indicating where your lure will land. Once that small circle hits the larger circle in the pond, release to cast your line. There is a small period of waiting until a fish bites. When it does, press to start the actual mini-game. A large thin circle will appear and start shrinking towards a smaller, thicker circle. When the two overlap, the thicker circle will turn green. When it turns green, press again. This may repeat several times. If you keep timing it right, you will be rewarded with a caught fish.

Simply repeat this 1800 times (or more if you catch seaweed or other items occasionally) and you should snag this very grindy achievement.