Hi Mervyn,
I must admit – I never knew about the Pozzani filter. I just had a quick look on the Internet (too quick, possibly) – I understand that the filter removes Chlorine/Chloramine (? – I am guessing so), heavy metals but does not remove the mineral content (e.g. Calcium/Magnesium) – am I right to think so? If “yes” is an answer, this is quite a good one – I use a DI filter which removes everything nearly as much as a RO filter for topping up evaporation in my tanks and sometimes for water changes (with mineral salts added, of course, to return the water to the state it can be used for an aquarium). In this case, of course, the “complete” fert is a must.
At least, we have just eliminated the “Nitrate” part of what could be wrong with your rainbow. The one which is left is the “fish equivalent of a human bruise” thing.... Although, I must admit, the pictures of your fish look a little bit more than I have ever seen with this sort of injuries, it may be just a severe case... Fungus infection “fluffiness” does look like a cotton wool with very fine strands sticking out – and I cannot quite see it on the photographs. Also, I cannot see a reason why a fungus would suddenly manifest itself in your tank – I can see it is very well maintained. There is a very slight chance of Columnaris being introduced with Nerites into your tank (snails are not affected but can carry the spores). However, I have a trust in your supplier – they could not possible send you snails from a holding tank where the fish are infected! If it was Columnaris, I would expect more than one fish being infected...
So, for the time being I would stick to just observing the fish and doing nothing but regular and slightly larger water changes (please, dose your “complete” fert). I hate uncontrollable medicating of the fish tanks – it does not help, it makes fish weaker, reduces their immune system capacity of fighting future diseases and treating with a wrong medication on impulse may even kill the fish (and then people say “oh, well, you did you best, a shame your fish did not pull through”). Treating a fish is only good when you are SURE you know what is wrong.
I will tell you my own recent story. One of my rainbows developed a white patch on her back. I was not quite sure if it was just that her scales gone damaged (rushing through a big bit of redmoor root in the tank) or whether it was a fungus (which would be REALLY strange as I did not add any new fish for months and the routine was always the same). The rainbow was otherwise very active, boisterous even and feeding extremely well. So, I left her to be. Her white patch have disappeared within 3 weeks getting smaller every day... So, I am going with a physical damage rather than a fungal infection.
I suggest you do the same at the time being. ONLY if you notice any change of the behaviour of the affected fish and the fact that the “white lip” is increasing in size may be the time to act...
Personally, I honestly think that your fish just had a very severe bruise – I am not claiming I am right but I would not rush into medicating as yet.
One point – I have mentioned this before on the “new” forum: if you want to spend some money, do buy a UV filter. This filter does not have to be switched on all the time (I’d rather not) but in situations like yours a UV filter is much much better than ANY medication! Also, a UV filter eliminates the need for a quarantine tank – you can add new fish straight in and even if they are infected with, say, white spot, you do not need to do anything – the infected fish will recover (mortality is close to zero) and the white spot will be eradicated without meds which are nasty and plus stain your carpets and your silicone! (I remember I had to “confront” Sue in that post about UV filters but I stand to what I said – confirmed by personal experience!).