If multiple readings are frequently taken, a polarographic meter is the preferred method. One possible exception is a computer monitored, intensive recirculating system. A variety of moderately priced colorimetric kits are available to monitor ammonia and niaite. Greatest precision is obtained with a spectrophotometer. Meters are available over a wide range of prices.
Performance depends upon daily calibration and regular maintenance.
Calibration is done with known standards called buffers. Colorimetric tests lack the precision of a meter, but are quick, economical, and adequate for field analyses. Conductivity meters are widely available and relatively inexpensive. If a Secchi disk is not available, you can use your hand.