Levels Rising... Please help!!

Have been getting spikes in my Nitrite, Nitrate, & Possibly Ammonia...
My test kit shows Nitrate near 40ppm, Nitrite lethally near 10ppm, and
I can't test for ammonia, but it smells strong in my tank...

It's a 10 gallon freshwater tank with (3) 2" adult Sword Tails, and
about 10 baby fry in a net... I just performed a full water change as
advised by PetCo, and cleaned out all the algae in the tank and things
were fine for about 5 days...

Also my Alkalinity (KH) is 40ppm and I know it's supposed to be 120-180
for freshwater fish, but the Pet store has no idea on how to raise
that...

I've used the AmQuel Plus that treats Nitrate, Nitrate & Ammonia for
the past 3 days and still it does not seem to be dropping anything....
Any advice would be greatly appreciated as I am hoping my fish don't
die...