Mystery Flashing / Flicking / Rubbing

Hi Guys

My 100L tropical tank is well established now at about 7 months. No fish deaths for some months. However, I notice that all of my fish occasionally rub, sometimes its days before I see one of them do it, but I'm concerned that something isn't right.

There are no signs of disease, infection or parasites. I treat them for ich a few weeks back (although don't think they had it) and also followed that with a treatment of Esha2000 which should pretty much deal with anything.

My tank always has some Nitrate present, usually rises from about 10ppm to 20ppm within a week. However, I do between 25% and 33% water changes weekly with Prime declorinator added to the new water before adding. I had a conversation with my LFS about the Nitrate and they said that anything under 40ppm wouldn't really affect the fish.

Despite the mix of fsh (which I have since been advised shouldn't get on), my fish are peaceful and look healthy. They received a varied diet and are all very active. No strange poos or behaviour.

I'm at a loss as to what is causing the flicking.

Further info ...

Temp 26C
PH about 7.2
Ammonia 0
Nitrite 0
Nitrate 10-20ppm
Internal Filter


4 tiger barbs
5 platy
2 balooon molly
5 neon tetra
3 dwarf neon rainbow
2 dwarf gourami
1 german blue ram
assorted live plants

How often, roughly, do you think your fish are flicking?

Could your ph be very low? My fish flashed during my ph-crash a few months ago! Don't know if it was related but after I straightened the ph out (adding crushed coral) they stopped their flashing.