turtle ageing

Question
I have an adult red eared slider and a small eastern painted (both rescues). they are in a 150 gallon tank. with 2 big basking areas and one small, extra one. i have a tetra fauna rock waterfall filter with another small one. we are getting an extra smaller one from a friend because we have lots of feeder fish for the large one. there is an area in the corner which has a place to hide(underneath the rock filter) in the other corner is a basking rock. the big turtles rock is in the middle with a smaller one on the opposite side. i also have a large rock are on the bottom. they get along grate and the little turtle usually likes to sleep with the big one on his rock (its very cute). but what i came here to ask was can i get another turtle? i would like a different breed and something colorful.i was thinking a peninsula cooter, but i saw that they get very big. i would not like to make the same mistake the slider's owners did. they had the poor thing in a 20 gallon tank! the small one named nub nub i saved from a snapping turtle when he was a hatching. he is missing a leg, but he is a very happy turtle. do you have any suggestions of what kind of turtle i get? or how long it would take a cooter to grow to that size so i will have time to get a bigger tank....and is my set up ok? thanks you so much for your time and advice! PS. i have a 15 gallon tank from when i was a starter.

Answer
Hi Lily,

I'm confused--I thought you only had a 40 gallon tank?  Did you read over the information I linked for you?  If you do have a 150 gallon tank, the filter you have is inadequate for a tank of that size, especially for a turtle tank.  You need an external cannister filter rated for at least twice the tank's capacity (so for minimum 300 gallons).  

It would help if you could post some pictures of your setup.  I really don't recommend getting more turtles unless you are 100% certain your setup is correct--which includes excellent filtration and a good source of UVB.  Cooters also get very large, and keeping an RES and cooter together would require a very big tank.  You can roughly figure a minimum of 10 gallons per inch of turtle, so for example if your RES is 10" and your painted is 4", you need a minimum tank size of 140 gallons.  An RES and cooter together could easily need 200 gallons or more (and keep in mind that means a bigger filter).  At this point I would recommend you stick with the turtles you currently have and work on making the perfect environment for them.