It's a done deal now. I removed all my plants (plastic & real), and the cave, then used a large fish net to scoop the gravel out. I changed 30% of the water since it was so cloudy by then. The I used a cup to add the new gravel to the tank. Smoothed out the gravel, replaced the plants and cave, turned the filter and heater back on, and after about an hour "voila" the water was clear again. I tested and the readings are fine. All the little fisheys are fine too.
Putting new gravel on top of the old one is a great way to retain the beneficial bacteria. However, if there is much color difference between the two gravel types, then you'll still have a tank bottom looking unnaturally. Sooner or later you'll have to vacuum your gravel and you can't do that effectively without stirring the gravel. Once you stir the gravel, it'll be possible to see the unnaturally looking gravel. Unless you don't care about that, another thing you can do is remove the old gravel, put the new one in, AND take some of the old gravel and scatter it across the old gravel - this way the beneficial bacteria from the old gravel will faster colonize the new gravel. If you don't want to mix the 2 gravels, put the old gravel in some aquarium safe container, and then put that container on top of the new gravel. When the bacteria have colonized the new gravel, all you'll have to do will be to remove the container
Well, I didn't put the old gravel back in, but I think that during the process of removing the old gravel, that most of the bacteria got swirled around that it was mostly in the water and off the old gravel being as the water was really cloudy during the change. I've been testing almost everyday since and all the water readings are perfect. The water cleared up about an hour or two after the change, so I believe that the bacteria was cycled thru the filter and returned back into the tank. I've been reusing the filter,(just rinsing off the gunk with the water I remove during water changes, and then putting the same filter back in to keep the beneficial bateria circulating).
I read about that in another post. But I don't know how long you can do this before the carbon is used up. I did come up with an idea about using an old filter with the carbon removed from it, for times when I need to medicate the fish. That way the water is still being filtered to remove gunk, but without the carbon. That keeps my tank from clouding up from removing the filter during medication.