We can also talk about test kits. I’m sure you’re using API test kits because that’s what most fish stores sell. But for anything other than ammonia they aren’t the most accurate. While you’re ordering the hydrometer pick up some Salifert test kits. They’re inexpensive, easy to use and accurate. You only need ammonia, nitrite, and nitrate for now. Maybe pH but API is ok for that. If you want to get everything you will ever need, you can also get calcium, alkalinity, magnesium, and phosphate but you won’t need those for a while. If you want to get just a few, grab phosphate, then alkalinity.
You won’t need the calcium and magnesium until you start keeping corals but alkalinity can be a good measure of the others. If it’s ok, the calcium and Mag will probably be acceptable.
I’m sure I’ll think of more. As you will soon figure out, you will save nothing by buying cheap. I have boxes of stuff that I bought trying to save money, that I eventually had to replace. It’s better to save up and buy better quality. Everybody buys stuff that they later replace because it was inexpensive and advertised as “just as good” as something else but turned out not to be. Everybody. Even those with huge stunning aquariums have closets of junk. I guarantee it.
Edit...I see I just contradicted myself.... expensive equipment, inexpensive refractometer. Geez... better idea... shop around. And ask questions.