I would be quite hesitant about using plastic test tubes for the testing. If you have a look at your booklet with the test, it probably gives warnings about the liquids that you add to the tank water in the test tubes. For example, the ammonia ones are acids (sodium salicylate, sodium hydroxide, and sodium hypochlorite) and are corrosive, so probably not the best thing to be adding to a plastic test tube. Also, the plastic may absorb some of the chemicals and affect your test results.
I'd say the safest thing would be to get glass test tubes as soon as you can if you want to continue the tests.