Next time you redecorate a tank, take the top 2/3s of the water out along with the fish. A lot of organic garbage gets stuck in the gravel and while that may be good for plants (in moderation) it can be very dangerous for the fish.

Fish have pretty much the same internal bones in their ear structure that we do. Obviously they don't have ear lobes, but water transmits sound very well. Their air bladder and whole structure is designed to pick up sound. Fish with lateral lines are even more sensitive. You may have been clobbering them with the noise.

Aside from the considerable danger to kids for banging upon aquariums, that is another strong reason for them not to tap on tanks. Probably somebody I know has asked them how much they would like a person tapping on their ears with a baseball bat. ;)

You will not likely get such a concentration of dirt in the sand as with gravel, though anaerobic spots can develop unless you have fish or snails which frequent the sand and stir it some. Siphoning off of sand is a little trickier than off of gravel. Run the waste water into a clean bucket dedicated to things fishy. (You probably knew this - in that case this is for someone reading along.) Then the waste water can be poured off, the sand rinsed and returned - as opposed to plugging a drain. :)

Some species are more comfortable with sand, though not necessarily your species. Some rooted plants have difficulty living in sand. Java fern on top of the sand and floating plants would do fine. If your tank can remain clean, your fish are healthy and you like the attractiveness of the sand, go for it. :)

Assuming the correctness of your water parameters, you are due (soon) for a partial change because of the 20 PPM nitrates. I get nitrates and nitrites confused too. :( Were you able to keep a box or sponge filter wet, so it could import the nitrogen cycle to the new set-up?

With that raphael catfish, do you get any guppy fry? How long have you had the guppies? Will adult guppies be safe with it when it grows up?

Do you have a cave (or chemical free flowerpot resting on its side) for the catfish? In cooler weather, it or the guppies could get burned resting on or by the heater.