At a walmart here they have mono's and scats for about 3$ each but there in freshwater and the staff there (who doesnt know anything) say to keep them in fresh water.Ive heard they need to be introduced to brackish as they age.If this true when is a good age?size to add the salt???thanks

You can actually start taking them to brackish any time and scats will do very well full salt (1.025) not sure about the monos tho.

I have a Mono that's been in saltwater for about 8 months now and when I got him he had already been in a brackish tank for about 2 month so he was pretty young. Acclimation Time has to be done over a few weeks move the salt lvl up very slow.

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Monos and scats are often sold as "freshwater" but will inevitably succumb to fungal diseases, and die early if they are not kept in brackish water, so always put them in brackish as soon as possible.
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you hit the nail on the head, just thought I'd share a little of my knowledge of these wonderful fish.

The only time that you will find scats and monos in freshwater is for breeding/spawning. After this most large adults will head back out into the ocean and the young will continue to grow in marshes, swamps, etc that are close to the ocean (BRACKISH water areas, nott even the young stay in fresh for long). You should start them in fresh water and the way I first moved them into the brackish setups they are in now is every day or 2 I would do small water changes, in a small tank I would only change about a gallon or so every time, larger tanks you can manage a little more, take it slow, use a refractometer to measure the gravity. A rough guess it'll be about half of the fresh water left in the tank and half salt when you get done, but be sure the check along the way.

Monos and scats have the potential to be quite large if given enough space, so be ready. I'm addicted to watching mine grow and am constanly trying to give them more room. You may catch the bug yourself... it gets expensiiiiiiiiiiiiiive.