Dear Peter, the best option ...
Published by Ciaran Gillen, Managing Director at BioFuture Ltd.
Dear Peter, the best option is likely to be test strips. Some potential suppliers are Merck, Hach, Macherey-Nagel, Fischer Scientific, etc. and there are a number of suppliers of test strips for water quality in ponds and aquaria. Check what is available and select what best meets your requirements.