Help with platy and cory!

Question
Hey there, can you help out??  

Recently (about two weeks or so ago) we bought three regular platys and
two albino corys, and I put them into my tank.  
My tank is around 30 gallons with a betta fish and three glass catfish
beside the newly-bought ones.  
The platys and all seemed to have been doing fine so far, but lately,
my one platy is bobbing on the surface, and he barely swims at all!  He
looks really sick, but I don't know what to do about this, as it's my
first time with platys.  
I'm a bit suspicious as to my fishtank's condition as of now, because
one of the corys also mostly seems to sit on the bottom of the tank, and
not move around too much.  

I don't know what's wrong, though!  I cleaned out the tank recently and
today, when I did a test, all of the levels were decent!  My nitrate is
around 40 ppm, but I've just done a partial water change last week to
lower that.  Otherwise though, nothing!
I don't however test for ammonia; could that be the problem??

Anyway, thank you very much for hearing me out.  My main concern as of
now is how to save my platy!  
Please respond ASAP, as I tend to fret over my fish a lot.  Sorry!

Thank you!

Answer
Hi Bluebetta
It could be ammonia in the tank.  How often do you do water changes?  I usually recommend vacuuming the gravel and doing a water change once a week, changing out about 25% of the water each time.  In a tank that's not overstocked(and if that's all you have in there, yours isn't) that's usually a good way to keep the water parameters in check.  I'd also pick up an ammonia kit too.  Your nitrates are a tad high, it used to be recommended a few years ago under 40 was ok, now they're saying under 20 is only acceptable, and I've even seen one fish forum where they were recommending under 10.....So, I'd try and keep them under 20 ppm, and the ammonia and nitrite should always be at 0 ppm.  Like I said, doing those weekly water changes will keep those levels or less.  That may be what's wrong with the platy and possibly the cory as well.  
Try doing another water change immediately if you can, and see if the platy perks up after that.  

This is my opinion here, but I've had platies, mollies, and guppies over the years.  I tend to do better with platies for some reason.  But I have noticed with all of them, that if I buy say, 4 or 5 of them at once, within a month or so, I'll only have 1 or 2 left that make it any length of time.  I hate to say they're not a hearty fish, because they can be, some I've had made it through some really awful mistakes I made.  One platy in particular I had made it several years lol.  But I think they're so inbred, and overbred, and they're so common, that it may have made them somewhat unhearty genetically speaking, if that makes any sense.

One thing with cory cats, they love to be in groups of at least 3-5 minimum.  And, make sure they're all the same "type", ie:  3 albinos, or 3 julii's, or if you have mixed ones make sure there's at least 3 of the same for each breed.  Try adding 1 or 2 more to the group and see if that helps.  Plus, your corys may be more active at night, or do you notice them active more after feedings?  I know mine go wild when I feed.  Sometimes they'll be sitting still, but usually not for very long.

Hope that helps, and let me know if you have more questions.

Christy