African Pigmy Angel gone hyper

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.

Not huge - 240 litres - which I think is about 55 US Gallons. The pigmy angel has lived happily in there for 5-6 months now. It took ownership of the space behind the rock work and until recently, seemed content to weave in and out of the reef grazing as normal. No recent additions to the tank in terms of livestock and environment has been extremely stable from a biochemistry point of view since initial cycling about 6 months ago - i.e. never any problems with the key parameters.