Click image to enlarge Scat (Scatophagus argus) By Tony Terceira.

Mono (Monodactylus sebae) By Tony Terceira.

A. When you consider that both the mono (Monodactylus sebae) and scat (Scatophagus argus) are brackish water fish that can easily be acclimated to the marine environment, it becomes hard to think that they could comfortably live in fresh water. While brackish water fish vary in their tolerance to higher or lower salinities, both these fish are rather hardy when considering the environment change. A couple of interesting things come up when we look at many brackish water species. At one point in their lives they will often tolerate fresh water, though as they mature they must be slowly moved into the marine environment. For both the mono and the scat it is possible to keep young fish in fresh water. The mistake that many aquarists make is that they acclimate the fish to fresh water and then leave it there long term without researching the fish’s need to move to a more marine environment as it grows.

It may be possible to mix the mono and scat with freshwater fish temporarily but not long-term, though I can’t really recommend it. Both the mono and scat require a large aquarium. They are both active fish so open swimming room is important. The maximum size for a mono is around 8 inches with a scat eventually growing to a similar size. A 55-gallon aquarium may be suitable for either fish species, though a larger aquarium is always better. It would be rather painstaking (due to fish size and aquarium requirements) to have to move the fish from one aquarium to another. Better success would be accomplished with having a species tank that would house the fish from the start.

Typically, the aquarist would slowly increase the salinity of the water as the fish grow. The eventual goal would be an aquarium made up of sea water density around 1.023. At this point hermit crabs, snails and other marine invertebrates could be added to the tank. Even compatible marine fish would be an option. I believe the excitement of watching a fish transition through various life stages (freshwater to marine) is what draws people to keeping animals like monos and scats.

In my experience monos have proved far hardier than scats. I found that scats are prone to ich. It is interesting to note that monos spawn in the high-salinity waters of the ocean. One thing to be aware of is that in the wild, monos consume a large amount of algae and plant matter. It is important that this diet be replicated in the home aquarium as it would contribute greatly to the health of the fish.

Overall, monos and scats are exciting fish that are very different from any other marine or freshwater animal. The unique stages they take when going from juvenile to adult make them highly interesting and I would assume for the prudent aquarist very rewarding. To have a marine aquarium that is stocked with fish that you worked through several demanding life stages would be rewarding and no doubt serve as a highly interesting conversation piece.

One thing that keeps many aquarists from keeping either species is both their large size and the demands they present as they grow older. However, I think that any aquarist could learn a lot about these interesting animals and aquatic ecosystems in general by transitioning these fish through their freshwater juvenile stage up to their adult marine stage.