Im not sure, but yes I am going to get a test kit, but I will also be getting plants to help my water quality and help the fish feel more secure. would adding plants be a mistake? Also is API the test kit that I should get? That seems to be the one that everyone has and since my lfs doesn't stock them I need to order them on amazon.
The plants won't hurt, they might even help a little bit but the test kit is a must have. API is the most popular kit here, buy one of them and don't worry about liquid carbon or adding ginger and things to food and such things yet.
Focus on the basics. Keep the water changes frequent as you can and if possible increase aeration as the liquid carbon does drop the oxygen level of the water.
How are all the other fish going now? I'm starting to think you got some dodgy angelfish from the shop. Even if that's the case, keeping the water fresh and clean for the other fish and adding some plants won't do any harm.