Nowadays, many pond dealers offer the service to test your pond’s water for free or at a reasonable price, as long as you “bring in” the water sample yourself. Now, in order not to “taint” the results, you have to “take” your pond water’s sample the “right” way: take a clean, empty mineral water bottle (never use the bottle of a carbonated beverage), rinse it for at least three times with your pond’s water (to remove any residues from dust, previous samples, etc.), then select a location within your pond that is sufficiently deep and away from any external “sources” of water so that you won’t be sampling “newly-poured” water. Submerge the water bottle below the water level and allow it to fill completely to the top, then “screw on” the bottle’s lid tightly to prevent leakage, and, for the most accurate results, bring the sample as soon as you can to your pond dealer.
Perhaps for even more accurate results, repeat the sampling from a different location in your pond and using another water bottle, so you’ll be bringing two “bottled” samples with you.