Hi FSN, that's really helpful. A follow up question...my sand bed is only 2-3'' deep. I wonder if I could funnel more sand in on it over time? Or should I vacuum it because it is shallow? Or leave it alone and let things take care of themselves?
Yes, you can add more sand over time, if you like. Recommendations I've seen (and followed myself) state adding no more than a 1/4" layer at a time, in order to prevent smothering some of the microfauna living in your existing sand.
I'm thinking that the sponge in my filter gets REALLY goopy...I had to scrub the bejeezus out of it to get some of the funk off. I don't know if swishing it in old tank water will clean it enough. Or, if I cleaned it too much in the sink! Also, when I replace the sponge (recommended every month or so on the box...aren't I losing all of the bacteria I'm supposed to want? So confused.
Some people that use those filter sponges (I do not) buy a couple spare ones and rotate them out. In the meantime, they heavily clean the one just removed from the tank. If I remember correctly, they even do so in a very mild bleach solution to help remove all the gunk and essentially sterilize the sponge (I encourage you to double check this with an internet search if you'd like to do it). I would think they rinse the sponge very thoroughly to remove any remaining bleach before returning the sponge to the tank.
When you clean the filter sponge in old tank water, you will certainly remove some of the beneficial bacteria along with all the gunk. That amount, however, should not be a significant portion of the total beneficial bacteria population in your tank. If you clean the sponge in the sink using tap water that contains chlorine, you definitely run the risk of killing off all of the bacteria on the sponge.
Most of the beneficial bacteria should be contained within your live rock and sand. Bacteria will populate any surface they can, which includes the filter sponge. If the filter sponge was a significant part of the total inhabitable surface area for the bacteria (or the only surface area for them to grow on), there would be more cause for concern. Since you have a good deal of live rock as well as sand, I wouldn't be concerned with the amounts of bacteria you are losing off the filter sponge from regular rinsings or switching it out from time to time with a completely clean sponge.