New Tool in the Arsenal

I was excited to see that my TDS EZ meter arrived today. I was pumped to try it out, and the early returns are in:

tap water: 303 ppm tds (consistent with what I research in September)
30 gallon: 543 ppm tds
5 gallon: 551 ppm tds

I'm not sure what level of trust to give these, yet. The specs of percent accuracy aren't terribly flattering, but I notice some consistency between consecutive dips at least (within 10 ppm or so). It kind of surprised the measurements in the tanks are so similar; for whatever reason I suspected that the 30-gallon would have a higher reading, but that was probably silly of me to think. After all, the 30-gallon gets a lot of fertilizers, but the light allows the plants to use a lot of ferts. In the 5-gallon, I add no fertilizers, but growth is slow as a result.