10-04-2009, 01:41 PM
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A 30g tank is not sufficient space for discus and angels, individually or together. Discus prefer warmer water than most tropicals, 80F or above as you mention. Suitable tankmates [in larger aquaria] would be cardinal tetras, or any of the quieter characins that can tolerate (or prefer) the warmer water. Active fish should be avoided. Once acclimated, discus could manage in neutral pH water, although spawning would probably not occur. The hardness is equally important. And where the discus were raised (most being commercially raised fish) would suggst the safest water parameters. Similar for angelfsh.
What fish tolerate in nature and in an aquarium is frequently not the same. As soon as we put any fish in a closed system we are subjecting it to an environment it would never encounter in nature. This has an impact on the fish's health--not necessarily negative, but an impact none the less. Behaviours and internal biological processes can be affected in many ways. If stress is involved, as it often is, the fish will require more effort to combat disease and parasites which in nature would pose much less of a threat.
Water quality also plays a large role. While the environment may not be ideal, having the water clean and stable frequently allows some latitude, though the side effects may not be readily discernable.