Our African Pigmy Angel started rushing around the tank a few days ago
and spending long periods just swimming up and down the glass at one
end. He stops to feed, and he seems to be eating normally. He also
looks normal physically and there has been no observed harassment by
other fish.

There is no trace of ammonia, nitrite, nitrate or phosphate. pH is
about 8.2 (that's all the test kits I have). Salinity is normal.

Changed carbon filter in case some other contaminant had found its way
in but no effect. Did a 25% water change. No effect either.

Only thing unusual that has happened recently is that phosphate
introduced from bad batch of RO water has been removed with NO-PHOS
from Advanced Aquatics. This is still in the filter.
Everything else in the tank is looking normal and healthy.

Thoughts?