When is it best to test water?

Hello,
May sound silly but not sure is water should be tested:

1st thing in the morning?
After first feeding?
Evening before last feeding?
After all feedings?

I am referring to 116L Discus aquariums getting usually 2 x 18L water changes a day (morning after feeding) and evening after feeding.

That is a very valid question.

The answer depends on which water test you are doing though.

I will not go through each test possible and say what is the best time, but will give you guidelines on what would be the ideal if you had the time and facilities to do it.
Noting that I would guess few people ever do do the ideal water test anyway.

Ultimately, you want to get an idea of extremes within the tank
eg
pH can swing values between day and night....so such a measurement should be done under full lighting and again under no lighting (meaning....leaving it a few hours after lights on and off to take the measurement).

for nitrogenous waste, you would be best to see what the highs (stress testing) and lows are in your system. So, a test an hour after feeding would be good and a test first thing in the morning before any feeding would be good.

In an ideal chemical water test, each test should be done at least 3 times and the average taken......but the test chemicals are not sufficiently accurate to warrant doing that.

Water testing should be done without delay.....and temperature is an important factor in water testing.....so do all tests at the same temperature as the tank water.
There is absolutely no point taking water to a local fish shop for testing pH or ammonia if the temperature drops in the test water.

Do not delay in testing water unless you are doing a specific stress test of the biologicalo system (which is not really something that people do).

ian