Okay while I have never done a turtle tank I may be able to shed some light on you problem. The undergravel plates won't help. The 29 gallon take is not what I would think to be an ideal tank for a turtle due to most of the size of the tank going height instead of length this doesn't allow you to put in as much water (Because it gets too deep for you turtle) thus you have less water volume and you have to change the water more often. The best advice I can give you is <p>A: External Filter -> more efficient and cleaning cycling the water versus an internal filter.<p> B: Buy a Python aquarium vacum cleaner cleaner, makes water changes a breeze (I hooks up to a sink faucet and vacums and fills from the sink no buckets or siphon) and since you are going to have to change water anyways you might as well make it easy. <p> C: Longer tank not as tall but greater length more volume of water. More time in between water chnges.<p>Hope this helps support your local (See Non Chain Pet Store) for more detailed explanations. I am talking from an aquarium background so take what I say with a grain of salt. Good luck let us know how things work out.<p>A link to an online supplier I have that might help:<p>
http://drsfostersmith.com/ Good selection, good prices.