The more fish you add the more robust of a biological filter your tank is going to need. They may eat any scraps or waste your turtles leave, but they are also putting off lots their own amounts. The best way to remove the waste the fish leave is with your beneficial bacteria. You also shouldn't have to move any fish if you are just doing a partial water change, 10%-20%. A bigger tank should be your next investment and then a decent canister filter, I personally recommend to stay away from Penn-Plax Cascade canisters, too much bypass. The bigger tank will also help with the smell because you will have more water to dilute all the waste.
Do the filters you currently have have any capacity to carry bio media?
One final recommendation from one new turtle owner to the other, read over the
Sand FAQ. You'll thank yourself for never having to take your turtle to the vet for impaction.
**Edit: I never had to deal with impaction, just was aware of the risk and decided to educate myself on how to prevent it.