First off I have to give you the disclaimer that your filters are undersized. Filter ratings are based on the bioload of a community fish tank. A single turtle produces MUCH more bioload than a community of fish. The rule of thumb is to take the filter's advertised capability and divide by 3. So for you a fluval 405 is rated for 100 gallons / 3 = 33 "turtle gallons". An eheim rated for 125 is really = 42 turtle gallons. 42+33 and you have enough filtration for a 75 gallon turtle tank. This is the biggest part of your problem. You should seriously look at selling those filters and upgrading to a single Fluval FX5.
If you have a good amount of substrate in your tank ie sand, you will get a decent build up of nitrifying bacteria on the sand. With that... if you have a dirty filter... take it apart and rinse it under hot water. Get it clean. Don't worry about the bacteria colony, it will re-establish itself fairly quickly from the substrate.
Also if you are seeing 6.0-8.0 Ammonia, are you sure your test is good? If you're using those silly dip strips please toss them in the trash. They are super inaccurate. Go to your local pet store and get an API test kit for ammonia. You can either get the freshwater kit (ammonia, nitrate, nitrite, and pH) or you can just get the ammonia test by itself. API and Sailfert brand drop test kits are very good. Test kits do have a shelf life after which they become inaccurate and well useless. If your test kit is old... toss it and get a new one. The $8 you spend on an API test kit might save you hours and hours of frustration chasing down a problem that isn't there.
Test your source water... your tap water before you put it in the tank. If your tap water reads 6-8 with the test kit... no amount of water changes will help you as you're just putting more ammonia in the tank.
Your vet is correct in telling you not to dechlorinate the water. Do not use any water conditioner.
For a brand new tank... you'll want to use some new activated carbon in your filter for the first day or so. The carbon will yank out any chloramine in the water supply. Chlorine we don't worry about.. it will evaporate on its own. Having the filters running and cycling water will speed up chlorine evaporation. Most municipal water supplies use chloramine instead of straight chlorine now anyway because it doesn't evaporate. (chloramine is chlorine bonded to ammonia).
Carbon can last anywhere from a few days to 2-3 weeks and then it becomes spent and needs to be replaced. In a brand new tank... there's so much gunk in the tap water that it only lasts a few days before it becomes useless.
After you've ran the carbon on the tank for a day or so, you can choose to replace the carbon with fresh carbon or switch to Purigen. Purigen is sythetic resin beads made by Seachem. Purigen will attract and grab much of the dissolved organics from the water... which will leave your water very clear and not smelling. Purigen can be cleaned and re-used. I clean mine monthly and replace it after a year.
Unless you have free floating sand particles or dirt from a planted tank in the water, you don't need polishing pads. The pads gunk up quickly and become a source for nitrate build up.
No matter what brand filter you have, the filter media should look like this (after the initial tank setup).
Following the water flow... the water should hit the media in the following order:
1. Coarse foam fiter (mechanical filtration)
2. Purigen or Carbon (chemcial filtration)
3. Rest of the filter filled with as much spun glass or ceramic bio media as will fit (bio media)
