You can get the API master kit at Kensfish.com for less than $18.
How have you been acclimating your plecos to your tank?
It really is important to know your readings. You should take water samples to the fish store to have them do it for you (as long as they use liquid test). Make sure they give you the exact numbers, then come back and let us know what they are.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~