Thanks for your replies
I dose Excel at the recommended dose directly into the tank - not in specific areas. So I can rule that out. I have never dosed peroxide so it cannot be that either.
For fertilisation - I mix dry salts in deionised water to make up a solution, which then I dose so many ml's in standard to EI dosing. Therefore, nutrients couldn't have caused this.
Maybe it's just, after two weeks, decided that it doesn't like the new water so it's being stupid and shedding...
Only time will tell I guess? I'll let nature take its course.
If anyone else has this problem, or has had this problem, let me know and tell me what you did, if anything, to solve it.