Hi, im buying a 200l tank fo my terrapins which should last them until they go into the pond im building for them. It is almost 4ft long. What would be the best filter i could use in this tank. Also should i use a heater, i was thinking not to so they will get use to cold water(for when they go to the pond all year round). If i should use a heater what one should i use. A link to the filter/heater would be great or the make and model name. Thanks.

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external canister filters would be the most effective in my opinion mate, Eheim are pretty good.

Definitely an external cannister filter, internals are practically useless for turts, you need something rated for at least 3 x the tank capacity, preferably more as a smaller volume of water needs more filtration to keep it clean.
Check out the Fluval 40-, Eheim Classic, and Rena Filstar XP ranges, they are all excellent filters with Eheim being the best quality. Shop around for the best prices, you can often find equivalent Eheims for virtually the same price as the other brands.
You said your YBS are nearly a year old, they shouldn't need heated water any more and as long as the tank holds around 20°C they'll be fine, you should get this temperature just from background room heat and heat from the basking lamp.

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