If you get your own test kits, they can be a bit costly. I have a Freshwater Master Test kit, a green box, by Aquarium Pharmaceuticals Inc.., which contains individual tests for ammonia, nitrite, nitrate, low pH, high pH. I've had it just over a year and despite doing weekly tests still don't appear to have made a dent in it; so it's well worth the investment.
Check out garden centre, by phone might be easier/saves you a lot of legwork and time, and see if they sell playsand rather than buying any through petstore (it usually works out cheaper for a larger bag than what you could get at the petshop).
Your gravel should be easier enough to remove without having to empty your tank (you could take out about 20% water and then remove the gravel, use a plastic garden handspade or kids spade (that hasn't been used in garden) or even a plastic jug comes in handy to scoop up gravel (my younger sister uses this method when she changes gravel in her fishtank every year or so - and there is quite a lot of gunk that builds up beneath it!). Once you've scooped out the gravel you could siphon out any gunky bits left on the bottom before adding your sand.
Also, don't be in too much of a rush to put your axie back in the tank because he looks better, give it another week or so at least (one of ours took over 6 weeks to start eating again)
