This test kit above covers the range of needs:
http://www.kensfish.com/aquarium-supplies/test-kits/api-fw-master-test-kit.html
I didnt notice what size tank they're in,, but 50% water change twice a week for a new tank, is too much. Is the water clouding up?
You probably should not do any wc's for two weeks, then start doing, 25-30% a week, after that.
It really depends on the bio-load, but for the tank to cycle properly you need a build-up of beneficial bacteria and that means letting the water sit a while.
A sponge filter can be good place to cultivate bacteria and just never rinse it under tap water. It really depends how much stuff you have in your tank. I put one in mine bc I took out all of the substrate.
Regarding their not eating. I would ask what they were eating wherever you got them.
Only put in small amounts of food daily until you see them eating. Then feed them whatever they can eat in about 2 minutes, once a day, is enough. Of course they can eat more but not necessary. Mixing up the diet is a good idea.
Try some frozen blood worms, or little pieces of fresh tilapia or shrimp.
Fish generally take time to acclimate to a new environment and can be very skittish, especially in a new well lit tank
Sometimes they reject food when offered, but once they see their preferred food is not coming, they will eat what you gave them, off the bottom of the tank, or wherever it settled, only you will no longer be watching by that point, perhaps/
I see this with my jags. The female will eat anything, the male will only eat (Xtreme Big Boy 3mm pellets), and blood worms. I can even drop a piece of tilapia in there and he ignores it. Just like he ignores larger pellets.. but once while I had them separated I noticed again, he refused the larger pellets when offered... but then ate them off the bottom of the tank some time afterward,, -as they were no longer there and he was alone. So they may be (probably are) eating some, even though you havent seen them eat.
Good luck!