Try the API Master Freshwater Test Kit - a lot of people here use it and it should last you about a year, so it's worth the investment knowing what's going on in your tank so you don't lose anybody else.
Hi Jinchi
It's good that you've added some live plants. They will mop up your nitrates and you will come closer to a closed eco-system in your tank.
You mentioned that you have coral in your tank. When I started fishkeeping I was warned off putting coral or shells in a freshwater tank. I was told (by someone quite knowledgable) that they added something to the water that wasn't natural in a freshwater tank, but that was over 30 years ago and I don't remember what it added. But it might be better if you took the coral out. Perhaps one of our water chemistry experts can fill in the why on this.