With EI dosing you are supposed to do a 50% WC each week. You may just be building up super high phosphate levels or something else that's being tough on the shrimp.
High PO4 has no impact on livestock.
Algae supposedly in natural systems, but not where high plant density occurs also.
You just get more weeds or plant species shifts.
I use 50% as just an example. It was never written in stone. Folks can use the dosing Cal from Wet and then see what the outcomes are over less/more % and more less frequencies and then ascribe a risk factor to those outputs.
If folks decide to modify, they can do a number of simple common sense approaches.
EI is suppose to be non limiting for 99% of folk's tanks. That's the idea.
Folks often do weekly large water changes anyway.
You can slowly trim the dosing rates by say 10% for 2 weeks observe, if everything looks fine, trim another 10 and watch closely.
Since you are at non limiting levels, what you are doing now is slowly backing off and watching the PLANTS/Algae.
Once you note a negative plant growth response, you bump back up to the last prior dosing rate, say it's 40% of EI......... for your tank, this is what is called the critical point, Cp.
It will be different for every tank.
Once there, then...........you can start trimming off the % or the frequency or both, for the water changes, going for weeks, months without one.
This is a better/wiser approach than starting at the lean limiting side because you do not optimize the CO2 due to dependencies with limitation of the ferts, plants stunted due to little ferts available.
You want to learn and know what fert dosing looks like independent of other interactions. This teaches you far more than fiddling between various co limitations.
If you take this same approach farther, you can remove the CO2, reduce the light and drop the dosing to once a week or two........about 1/10th to 1/20th EI.
Then do a water change maybe once every 6-12 months.
Or modify it to add Excel and do them say once every 3-6 months and dose about 1/5th to 1/10th EI say 2x a week.
This stuff seems very common sense to myself, but I think folks read this and think it's set in stone.
You can modify it if you wish, that was the original idea really, or just keep doing it.
I do a version of non CO2 above on one tank.
I do the modified EI on this tank, basically more PO4 and dose more tracesAnd I do 2x a week water changes since I trim and garden frequently and need to clean up more:
And with this tank, I only change the water once a month sometimes a little more, but not much, I dose about 1/2 to 1/3rd EI.
But it has 1/3 the light tank above has and it rarely even gets anything uprooted or mucked up, gardened like the 120 gallon
The point is that some common sense should be applied, no one method will be all things to all goals. Any method that makes such claim is full of horse feathers.
I think this comes across in person well, but written in books or on the web......not so much.