[SeanB (OP)]

A question I am asking. I use white natural colour nylon ties in my fresh water aquarium, and in general after a few weeks of being in the water they change colour from white to a pale pink, and seem to stay that way.  Clear acrylic parts of the filter do not change colour, and only the nylon ties, and do not change colour back when removed and left to dry. Even those left in plain water do tend to do the colour change, noticed on ties that were outdoors where they sat in rain water. But pretty much all of them change, irrespective of brand, just have to be plain nylon ties.

[paul cotter]

Anything that is outdoor is subject to UV degradation and susceptibility will vary depending on the type of plastic. As regards the aquarium I just don't know, are any chemical additives used in the water?

[SeanB (OP)]

No, no additives, and not exposed to UV light at all, the tanks are indoors, and direct sunlight only hits half the tank in the afternoon, through 2 layers of window glass, and the pink is even, on all of the tie, even that side that is shaded. Water is plain tap water, run through a ion exchange filter and a charcoal filter, just in case there is any heavy metal or mineral contaminants in the very soft water here (though it does have asbestos in it, because there still is around 1000km of asbestos fiber cement piping all round in the ground, and the HDPE pipeline project sort of fizzled out after getting to only some households), and no other additives in the fish water, as that is there for the cat to drink.

Did take a plastic bottle and put another tie in it, and filled partly with water, to see if it is something in the fish food, though ties used outdoors in the wet also go that pink.

[Bored chemist]

Plenty of microorganisms produce coloured compounds which protect them from UV (and may have other purposes too).
Those will leach into the water.
If they are absorbed by the nylon- which is plausible, it has a structure that vaguely mimics proteins- then the nylon will be discoloured.

[Petrochemicals]

Fish food, bacteria ?

[SeanB (OP)]

Well, a month later, and I had an experiment running. the reddish tie is one that was in the tank, for around a month, and the white one is in plain water, the type i use to top up the tank, and it has not changed colour, despite being exposed to sunlight and water, like the red one above it. definitely something in the tank, though ones outside in rain puddles on the roof also did this, so likely in the tank from the food, flake food, that feeds the fish, and outside likely from leaves and such that sit in the water.

[alancalverd]

White nylon is not pure polyester, which is generally brownish and semitransparent (think nylon stockings and early nylon military parachutes). Not sure what the colorant may be, but titanium oxide is a common white pigment and whatever it is may be attractive to red bacteria and moulds.   

[Bored chemist]

https://en.wikipedia.org/wiki/Phycoerythrin
Maybe?