Just out of curiousity why? A 29 gallon tank without a filter sounds like it could pose a whole bunch of problems. My two main concerns would be as follows:
1)Aeration and water movement. A filter does both. By breaking the water surface tension a filter allows oxygen to enter the water column. I would think that the water would become very stagnant if it is allowed to sit for too long. How long is too long? I don't really know. I would imagine that it would depend on the amount of fish in the tank, and the bioload that is present. I would venture to say that if you were commited to doing frequent (and I would think very frequent) water changes than the stagnicity (if that's a word) could be avoided.
2) How else would cleaning and filtration be accomplished? Obviously without a filter I would think that if frequent water changes are not done, then matter would accumulate much quicker with out the presence of a filter. I am not saying that a filter eliminates matter from the tank, however it does collect it, making cleaning, and subsequent removal easier.
I guess I really haven't answered your question directly, but I hope you can see that I would really stress the importance of a filter, and the benefits of one, no matter what type of fish are present. Hope this helps.
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