Toadstool not growing

[josh-jonees]

Hi everyone I'm looking for advice on my toadstool, brought it as a neon green 2 inch frag, but its gone a horrible purple colour, its never grown and polyps only show a tiny bit. I've had it for 18 months now and its so frustrating, tried all flow options, hi and low in the tank with no luck. Anyone know the simple answer to this colour change and why its shrinking?

Its the only coral alongside gsp that I cant keep.

2 year old reefer 250
2xhydra 26hd - BRS AB+
Chaeto refugium - kessil h80
ATI essentials pro
KH 7.5
CAL 430
MAG 1290
PO4 0.04
NO3 0.02

Purple tang - 2 ocellaris clowns - chalk goby - coral beauty - bicolour blenny - scooter blenny - peacock wrasse - carpenters flasher wrasse - royal gramma

 

[amazongb]

Might be due to low NO3.. might try boosting it up..