I used a cheap USD20 7 in 1 "Pen" Tester to start off my measurement journey.

What I want to do was simple:

1. Ensure low build-up of organic material (low TDS; my proxy for ammonia-related organics)

2. Ensure good amount of dissolved oxygen (mainly for the fish; proxy by low ORP)

3. Ensure good pH levels

4. Ensure water is clean (low or healthy EC range)

I will be taking daily readings and monitoring plant health.

These are ideal ratings for freshwater aquaculture systems that each parameter my "Pen" Tester measures:

pH: 7 - 8.5

EC(μS/cm) : <1500

TDS (ppm): <400

Salinity: <0.05%

ORP (mV): 150-250