TDS meters are relatively cheap. It’s the ph meters you don’t want to cheap out on.
https://www.amazon.com/dp/B00PEDHSQC/ref=sspa_mw_detail_5?psc=1&pd_rd_i=B00PEDHSQC&pd_rd_w=eceGk&pf_rd_p=f32edc8b-35a7-4ea3-a797-581b57f69242&pd_rd_wg=oxbYt&pf_rd_r=02HW9Q8KH6E60GSC6245&pd_rd_r=3ccb8234-49cf-42fb-8b20-c3e4625d613d&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyUlBOTUswVDIzUlNFJmVuY3J5cHRlZElkPUEwNjkyMTU2M1JaTzZHVFFYQktQSCZlbmNyeXB0ZWRBZElkPUEwOTEzMTg4MTI4Ukk2WTNQNURPRyZ3aWRnZXROYW1lPXNwX3Bob25lX2RldGFpbF90aGVtYXRpYyZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=