Turtles/terrapins and fish don't go well together in a small tank.
1. Terrapins need shallow water and an 'island' to rest and some good sunlight during part of the day to bask. A small tank will not allow you the space to create the shallow area and basking area, plus a deeper area for the fish. The heat from direct sunlight may make it too warm for the fishes.
2. Terrapins might try to eat the fish. Even if they never catch them, they will stress them.
3. Terrapins food and waste create huge messes. A small tank will be high in ammonium/ammonia from these products decomposing. These are toxic to fishes... if it does not kill them, it'll make them very unhealthy. Frequent water changes will stress the fishes too.
You can get tanks in any of the decent fish shops. All kinds of sizes. Which area do you live in? The members will be able to suggest some good ones near you.
What size are you looking for? Are you setting up a bare tank, a planted tank, or a terrapin tank? All these will affect what you need to get, like how much lighting, if necessary, what kind of filters, additional equipments, etc.
Bookmarks