Frequent water changes are widely hailed as a key variable in the well being of aquarium fish, especially cichlids. However, for at least one species of angelfish, fewer water changes may actually be beneficial, especially if your angelfish population is somewhat dense.
According to an article published in the January 2018 issue of Applied Animal Behavior Science, frequent or large water changes in aquariums housing Pterophyllum scalare may actually increase con-specific aggression in this species. The authors posit that water changes modify cortisol and other chemical levels that these angelfish use to establish social status. Frequent and/or large water changes dilute these chemicals, which effectively disturbs the baselines that set the statuses thus requiring that the angelfish re-establish them.
Less water renewal reduces effects on social aggression of the cichlid Pterophyllum scalare. Gauy, Ana Carolina dos Santos et al. Applied Animal Behaviour Science (2018). Volume 198, pp. 121 – 126. https://doi.org/10.1016/j.applanim.2017.10.003.