If it is hydrated lime it is extremely fast acting lime.
If it is hydrated, the danger is changing the water pH too quickly. Fish do not take to fast pH changes and it can stress or even kill them.
If you choose to use such lime it would have to be done very carefully in small amounts at a time to avoid killing fish.
If on the other hand it is just ag lime it would be fine.
barn lime Does not really say much on the bag. I would think if it is hydrated lime it would have a warning on the bag about not getting it into the eyes. Hydrated lime is pretty caustic.
Edit: If you read the reviews, it appears it is just finely ground limestone. Hydrated lime might be dangerous to have around the animals???
