If I contribute my part, would you be willing to follow all suggestions provided by our fellow members?
Here is my opinion.
Gar-Strongly recommended to be removed.
Senegal bichir-A 15 gallons is okay as grow-up tank but be sure to upgrade once he reaches six inches.
Peacock spiny eel-Strongly recommended to be removed. Have you tried bloodworms and other meaty stuff? You must move him in a 40 gallons tank minimum.
Turtles-Strongly recommended to be removed. Do they have basking area in their 5 gallons? Sorry but they should be removed as well or better yet, place in a pond with basking area and barricaded to prevent them from escaping.
Molly and Dwarf Gourami-They will be fine in a 15 gallons but should be isolated from the bichir, gar and eel.
Blue zebra-Strongly recommended for removal. They are not generally good community fish.
Crayfish-Strongly recommended to be removed. Incompatible with fish. Either your large fish eat him or the crayfish ambush and attack all your fish.
Common Pleco-Strongly recommended for removal. 100 gallons minimum.
General lesson learned:
Research further before you buy. If you cannot keep the fish for a long time, get rid of it and stick to the ones that can be accommodated by your budget and tank space.