Toxotes microlepis can live in freshwater and I think are more suited than any mono...whenever I see monos for sale in freshwater they always have something wrong with them. If he was to go for monos, he would need to make sure that GH stays hard.
Probably the same deal with the archers though.
If Loony Fish is serious about keeping archers, he would be best to try keeping one in a tank with the same water first -should be no problem if he can afford to build, heat, filter and stock a 900gal pond anyway
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It just saves loads of archerfish getting ill in the pond - if the fish doesn't do well in the tank, just convert the tank to brackish, much easier to achieve than with 900gal.