Freshwater fish.

Freshwater is where the hobby starts for most aquarists. The trade catalog is enormous - 108 species in Fast Aquatics' database alone, drawn from cichlids (American, African, dwarf), tetras, rasboras, barbs, danios, livebearers, killifish, catfish (cory, pleco, otocinclus), loaches, gouramis, rainbowfish, eels, bichirs, arowana, and pufferfish. Most freshwater fish are now commercially aquacultured, which means stable parameters, predictable behavior, and far better disease resistance than wild-caught equivalents.

The key parameter for freshwater is hardness, not temperature. Soft-water species (most South American tetras, discus, apistogramma, ram cichlids, Caridina shrimp) need GH under 8 and KH under 4. Hard-water species (African mbuna, livebearers, mollies) need the opposite - GH 8-15, KH 6-12. Mixing soft and hard water species in the same tank stresses both populations and limits breeding.

Beginners do best starting with tetras, danios, livebearers, or peaceful catfish (corydoras, otocinclus). These species forgive cycling mistakes, eat readily-available foods, and rarely die from common keeper errors. Cichlids of any kind are intermediate-to-advanced; arowana, large cichlids, and stingrays are advanced-only and require 200+ gallon systems.