You need an external canister filter on a turtle tank, rated for at least 3 x the volume of water you actually have in the tank. e.g. if you have 100 litres of water, you want a filter rated to deal with 300+ litres.

Internal filters cannot hold the amount of bio-media required to cope with turtle waste.

Get a canister filter and fill it full of bio-media, I find Alfagrog to be the best.

Maps and YBS are strong swimmers so even as youngsters should have as much water as possible (deep) to utilise.