I know that conventional wisdom says it's good to have rocks in the tank/pond for bacteria to grow upon, but since I removed the (large) gravel from Al's tub, his water is a lot cleaner. Smutz used to get all mixed up in the gravel and it was hard for the filter to pick up, and for me to clean out when it was time to change water. Now that the rocks are gone, smutz is not collecting at the bottom of the tub anymore; the filter must be picking it up.
I also think that being outside is keeping his water cleaner- between the algae and the sunlight breaking things down, the ammonia levels are very low and I am down to doing partial water changes once a week or so and a full change maybe once a month.


