I have a 30 k gallon in-ground pool. I have had an issue with small piles of debris on the pool bottom under my 2 returns and the piles accumulate rapidly within 1 hour of running the pool. A large hand full per hour. When the pool is not running, no debris can be seen. I had issues with chipmunks and could hear them knawing under the concrete deck last fall. We also may have debris in the bottom drain. Could a small crack allow dirt into the lines?...I observe no bubbles. Here is a close up of debris..looks like larvae: This pike accumulated within 2-3 minutes. Larvae is very small- this is a pool broom stick net.