What snake said
I was thinking about adding some angelfish because they live together in the wild and shole together
Your first priority *has* to be to get 2 more discus or to get rid of 3 (the first is much better).
Your second priority should be to read about cycling a filter (for example, articles 1, 2, 4, 5 and 8 at http

/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/ ) because I am starting to get the feeling that your filter is uncycled, especially as you asked about what water parameters are, and an uncycled filter will probably result in the death of the discus unless you intervene.
Your third priority (after your filter is fully cycled, which takes at least 4 weeks and normally 6-10) should be dithers because without dithers, it's very possible for your discus to not act naturally.
Your fourth priority needs to be to upgrade to a tank they can have for life (at least 48l*24t*18w inches).
Only after all of this should you even consider any new fish. Also, discus and angels do not shoal together, but they do dither for each other to some extent. And another thing, as with discus, you should only keep 1, a pair of or 6+ angels, which means you'll need a bigger tank to support all these fish.