+1 for neilanh...
I ran a power head in a 29 for years, until I finally upgraded to a larger tank. A suggestion...if the undergravel plate has more than 2 openings (mine had 4), put the tube and power head on a hole that is not to the extreme left or right. In other words, if there are 4 holes, number them 1-4 from one side (or the other) and use either 2 or 3...not 1 or 4. That way, the pull from the power head will be more evenly distributed. If you are running 2 power heads, then use whatever holes work best with your landscaping...that much "suckage" will do the job regardless of where you put them.
Another thought, when you put the caps in place, make SURE they will stay attached. I put a small dab of tank-safe silicone sealer on the inner edge of each of mine before popping the cap on and letting it cure for a few days. Not enough to be permanent. I wanted to be able to pull them off someday, if I had too, but enough to be sure that a foraging fish could not knock one out of position.
Many years ago, I had a cap come off during a gravel cleaning. The powerhead was off, and I just happened to be looking right at the opening when a small cory went zipping down the hole! One blink and I would have missed it! Something similar could happen if a cap ever comes loose while the powerhead is on...except in that situation the fish might not survive.
