I have never found test strips to be accurate ! Your results will be very different with a good kit ! If you could down the road a larger tank will be / IS easier to control water perimeters , with the high maintenance for a large turtle you tank does not have time to ever cycle. Larger water volume can / will help in that area.
Why not carbon as Steve stated , it does the job as long as you are good when it needs to be changed ! About every 2-3 weeks depending on tank load. Conditioner has some bad chemicals. Seen some next step to use more chemicals to treat the chemicals they put in , not me , my opinion only.
Please read API's own MSDS and they are one of the good company's. (API is Mars Fishcare Inc.)
http://www.redearslider.com/~res-mod/ve ... 122309.pdfRead all you will see in there words----SODIUM THIOSULFATE : Very toxic to aquatic organisms / YES
Carbon sounds good now ! You read and you decide...