What should i buy to purify my water?

[megs_clark]

As soon as i find a tds reader ill post my reading and then mabe i can get a better idea of what i need to buy. Thanks for the opinions so far :O) so many options makes this all confusing sometimes,
I can say to a earlier post i really dont think im over feading. I take the frozen food, a tiny bit, put salt water in it to defreeze it, dump out the water and use a tooth pick to pick the food out and put in the water. The fish pritty much eat it almost before it hits the water, and i watch to see if any sink and it never does. Then i turn on the pumps. I do use a medicine dropper 2 times a week to squirt the brittle star so i guess that might be contamanating the water a little, but i use so little i thought he must be munching it all up right away?