not for long as it would starve ... but say his ro/di topoff water has phosphates ,, it would do just as you suggest, now wouldnt it... i bet his tds meter is bad and his ro membrane needs to be replacedActually, it could be, if the algae has consumed it faster than it's being added.
hard to really tell, i would definately be checking to make sure all paramters are really in line..
for all we know his salinity is 1.022... or 1.026 could be lots of things.. i tend to try to try to make sure any numbers being used are at a known baseline first.