Scott,
I baste my rock every week the day before water changes. Nothing happens except that some of the crud actually makes it into the foams in our mechanical filters
That's why cleaning the foams on our weekly water change day is so important becuase it exports the crud.
In your case, most of the crud has already degraded into nitrate and phosphate. But don't let that stop you from de-crudding your tank with the method I described.
Using an AC500 might be over kill due to the amount of flow it will add to your 20g tank. Maybe an AC300 might be more in tune with the size of your tank. Don't use carbon in it, use foam. The foam will collect the crud way better than carbon and you can easily clean the foam, then begin the basting cycle again. And make sure you clean the foam as soon as the tank clears so the crud is exported asap.
When I used this technique on our first tank, a 75g, I basted that poor tank sometimes 4 or 5 times a day. Even basting into the sandbed did not cause any problems, and of course, that's where most of my cyano was. Basting the sandbed broke up the cyano, sent it into the water column along with LOTS of crud, and all was sucked into the foams. Once I realized how safe it was to baste sand and rock, I made it my mission to baste and clean the foams as often as I could
Since that bad cyano outbreak in our 75g years and years ago, I always baste my rock (and sandbeds, when we had them) before every weekly water change.
HTH
