The phrase "although at a decreasing" is correct and usable in written English.
It can be used to indicate that something is happening or being observed, but the rate or amount is diminishing over time.
Example: "The population of the species is increasing, although at a decreasing rate each year."
Alternatives: "though diminishing" or "albeit declining".
Meanwhile, the naphthalene and 1-methylnaphthalene plumes continued to advance and increase in mass over the observation period, although at a decreasing rate.
The slender and long spans in combination with the fact that permanent loads are only partially compensated for by prestressing are reasons for the large deflections that increase during the life time of the bridge, although at a decreasing rate.
The thickness of the diffusion layer increases with time, although at a decreasing rate; depths of 1 to 2 millimetres (0.04 to 0.08 inch) in 4 to 16 hours are typical.
The remitting likelihood also rises with the migrant's age, although at a decreasing rate.
After the precipitation of halite, the sodium concentration decreases, but chloride concentration continues to increase, although at a decreased rate.
B used, tip cells were able to generate lamellipodia that provided the driving force for cell movement although at a decreased velocity.
And also in state-run institutions it is easy to get a nonpermanent job at the moment--although there is a decreasing number of jobs, the number of candidates is decreasing faster.
Two cohort studies in children were similarly inconclusive, although both hinted at a decreased ability to control parasitaemia with more advanced HIV-disease.
Nevertheless, the respective distribution is in Hardy-Weinberg equilibrium, although with the LAB-specific allele at a decreased frequency.
In the case of LRRK2, 4E-BP phosphorylation can occur when complexed to eIF4E although at a slightly decreased rate.
In other words, the larger the family-provided insurance premium paid today, x, the greater future coverage will be (i.e. g ′ (x) > 0), although the increase will take place at a decreasing rate (i.e. g ″ (x) < 0).
