Since you have a real need to "keep that water you bought" focus on getting your FC (Chlorine) levels correct and then work on your other levels (ph, TA, CYA). The green monster is what you want to avoid and CL will keep it away. Bleach/chlorine is your friend. If you have water, you need bleach.
While it sounds extravagant, the recommended test kit will save you tons of time and money in the long wrong... and toms of heart ache in the short and long runs. It really does cost less than most trips to the pool store.