I think fish don't drink,
if the fish drink, they will bloat,
just according to my observation,
fish will still drink even though the amount is very small,
because after all, life still needs water.
I'm not an expert on fish but if you think about it,
fish should have a water filter system so that not much water enters the stomach.

It is a very interesting and important question, well fish do not drink water in the same way that humans and other land animals do. Unlike mammals, fish are able to obtain the necessary moisture they need to survive from the water in which they live.

Fish are able to absorb water through their skin and gills, which are highly permeable to water. The gills are especially efficient at extracting oxygen from the water, but they also allow for the absorption of water molecules. This allows fish to stay hydrated without ever having to drink water in the same way that mammals do.

However, in some cases, fish may consume small amounts of water from their environment through their mouth. For example, some species of fish live in environments where the water is highly concentrated with salts and minerals, so they need to drink additional water in order to maintain their fluid balance. 

In conclusion, while fish do not drink water in the same way that mammals do, they are still able to stay hydrated through the absorption of water through their skin and gills.