| Mai-Ndombe dwarf cichlid Nanochromis transvestitus |  |
| Marbled hatchetfish Carnegiella strigata strigata |  |
| Metallic livebearer Girardinus metallicus |  |
| Midget suckermouth catfish, Dwarf sucker catfish, Oto Otocinclus affinis |  |
| Millions fish, Guppy Poecilia reticulata | .jpg) |
| Molly, Liberty Molly, Black Molly, Short-Finned Molly Poecilia sphenops |  |
| Moon redhook, Red finned silver dollar Myleus rubripinnis |  |
| Moonlight gourami Trichogaster microlepis |  |
| Mosaic corydoras Corydoras haraldschultzi |  |
| Mosquito fish, Dwarf livebearer Heterandria formosa | |
| Megalomphodus megalopterus Black Phantom Tetra |  |
| Melanochromis johanni Johanni Mbuna |  |
| Melanotaenia boesemani Boesemans rainbow fish |  |
| Melanotaenia fluviatilis | |
| Melanotaenia papuae Papuan Rainbowfish |  |
| Melanotaenia praecox Dwarf neon rainbowfish |  |
| Melanotaenia trifasciata Jewel rainbowfish |  |
| Mesonauta festiva Festive cichlid |  |
| Metriaclima estherae Red zebra |  |
| Myleus rubripinnis Red finned silver dollar, Moon redhook | |
The biggest killer of tropical fish is overfeeding. Uneaten food decays and pollutes the water, even with adequate filtration. Adult fish should remain unfed one day each week, and almost all commonly available species will be completely fine if unfed during a two week vacation