There's a problem with very low levels of NH4 and uptake testing.

You cannot do it with most methods.
Resolution at low levels really does not tell you much/cannot tell you much.
Also, NH4 is taken up very rapidly by many biota and cycled into various groups.

The only way to look at this is to use stable isotopes like N15H4, or N15O3 and dose to a controlled system and then measure where it goes. No one has done that yte.

I suggested it many years ago.

Water changes do a few things, increase circulation to slow moving areas, adds CO2, O2, disturbs existing algae, removes spores, organic fractions of N etc.
Light is also a factor, more = less stability in terms of plants vs algae.

NH4 is linked to several other factors, it's a path/net, not one cause.

Folks had added NH4 and done fairly well with it, but daily dosing, lots of water changes have also been part of the game as well.

Non CO2 methods should not induce algae unless you really did something wrong.
Generally too much light, doing water changes etc, not enough plants etc.

Many aquatic systems have very low, but constantly resupplied or such large volumes, that plants cannot remove it. Both for NH4 and NO3.

Then you have sediment based NH4 sources as well.

So you can add it to the sediment and then NO3 to the water column.
Which is what ADA seems to do. Where it's added also can make a difference in growth.

Regards,
Tom Barr