Your 36 gallon tank is too small to comfortably house the larger species of Spiny Eels (Mastacembelids) at adult sizes. The Peacock Eel (Macrognathus aculeatus
), Striped Peacock Eel (Macrognathus siamensis
) and Zebra Spiny Eel (Mastacembelus zebrinus
) would work because their maximum size is generally under 18 inches. Bear in mind that these fish are substrate burrowers so a fine-grained gravel or sand is best to use on the bottom. Some sort of cave (or piece of PVC
pipe) would also be acceptable as an alternate hiding place for them.
The Ornate Bichir (Polypterus ornatipinnis
) will eventually grow too big for a 36 gallon tank. They get to about 24 inches or so but aren't as flexible as the Spiny Eels and need a tank that is wider than a standard 36 gallon. As an alternative, the Senegal Bichir (Polypterus senegalus
) and the subspecies of Polypterus palmas
would be acceptable for your tank.
If you want a shoaling type of fish that is on the carnivorous side you might consider Exodon paradoxus
- sometimes called the 'Bucktooth Tetra'. They are pretty fish and very active but are best kept in groups of 10-15 so their aggression is spread out among individuals. They do not do well in a 'community' tank with other types of fish.
Among my personal favorites are the Bush Fish (Ctenopoma
species) from Africa. These are generally ambush predators and some of them are quite beautiful in coloration and pattern. The majority of them are not safe to keep with any fish they can swallow (and the mouth on some of them is quite capacious).