Anytime you feel the need to test PO4 or NO3. o a water change, cleaning, and do at least 50-60% and then dose the nutrients back, then you are spending your time in a more productive manner.

Testing is a guessing game, you know why and what you did to get the plant to stunt.

Generally it's neglect, and..........that's all the test kit will tell you if it's accurate in this case with the Ludwigia if it's NO3/PO4.

But you can already resolve that without a test kit by the water change.
Then it's just a matter of CO2.

You know it's low or CO2, so why not just start doing the work rather than tinkering and testing?

Don't beat around the bush, take care of the tank, do frequent water changes/dosing and the plants do very well, algae hates it.

This is the same old thing 100X.
I'd do this and then if you feel the need, then test.

Test kits lead folks astray often times or give readings that send the aquarist off some time wasting road. They think their NO3 is too high, when it's really too low and the same with PO4.

If you think the NO3/PO4 are too high(you need to know what range that is, it's very high BTW and does nothing to plants even at 10ppm /100ppm PO4/NO3) do a large water change, that takes care of that fear.

Now dose 1ppm PO4 and 10ppm of NO3, now you know you have enough of each but not too much.

Regards,
Tom Barr