You can do a complete water change and sterilize the stones by soaking them in white vinegar and then rinsing well with hot water.  Also, if you do a complete water change, you need to add more ZymBac (2 TBSP).

You can purchase Water Garden re-fill supplies on Kordon's website here.

Alternatively, you could bring a sample of your tank water to Petco and have them test it for free. If it tests normally, no need to do a water change. Whatever the growth is isn't impacting the quality so it won't effect your fish. If the water is testing at a poor quality, I would go forward with the complete water change. You can also buy aquarium home testing water strips at any major retailer.

Hope this helps and let me know if you have any other questions.