I'm going to disagree rather strenuously with Colin_T here, so forgive me.
Monodactylus should not be kept in freshwater long term. Part of the problem is that there are two very similar species in the trade, one of which, Monodactylus kottelati, is rarely, if ever, found in completely freshwater conditions. Since distinguishing the two is very difficult, if you happen to have Monodactylus kottelati in your batch of monos, keeping them in freshwater will severely stress them. Looking around the UK market, I'd guess about half the monos are Monodactylus kottelati, and this is why some people find they stay small and others find they get big -- Monodactylus kottelati is only about half the size of Monodactylus argenteus when fully grown.
Furthermore, they can be adapted between freshwater and saltwater (let alone brackish!) within minutes. These fish live in the intertidal zone and have no choice about the matter. When the salinity changes with the tide, they have to adapt. Frank Schaefer in the Aqualog brackish book argues that you can literally dump Scats from freshwater into saltwater without causing them any harm. I'd perhaps be a little more gentle with monos because they are rather temperamental fish, but his point is valid. If you need to adjust them to brackish or saltwater conditions, simply use the drip method (as done with marines) across, say, 30-60 minutes.
Monodactylus move in and out estuaries all their lives, so they aren't that fussy about salinity. There's no need to keep them in a marine tank. But I'd still never recommend anyone keep them in freshwater. Aim for at least 20-25% normal seawater salinity (that's upwards of SG 1.004-1.005). Your fish will be healthier, less nervous, more outgoing and more colourful. Also ensure the pH is high (ideally around 8) and well buffered by providing ample carbonate hardness. A protein skimmer is well worth it with these fish if you want to minimise water changes. They are surf zone fish and super-sensitive to low oxygen tension, and anything you can do to keep the redox potential high will help in the long run.
All this said, the only instances of Monodactylus spawning in captivity have been when they were kept in freshwater conditions. But still, they just don't do that well in freshwater tanks most of the time.
Cheers, Neale
