So long story short, corals started to look a bit worse for wear, some stripping and others closing up, while others looked fine... Checking the params all is ok, calc/alk/mag, slightly high nitrates around 15, but in general everything looks to be on par, thinking that may be I have mixed up or got a dodgy batch of dosing, I switched off the doser for a while and did a couple of large water changes over the next couple of weeks, still no change. Changed out 2 T5 tubes, thinking that perhaps one of these had gone south, and 10 days later still no change. confused about why this was happening, I decided to do another large water change, when it suddenly dawned on me that maybe TDS could be out, Testing it with a tds pen it revealed that it was running around 3, but not overly convinced that this was accurate, I purchased an inline tds meter, to discover that the inlet was running at 35/36 and the outlet was hover around 16/19! So a quick change of DI resin, and a new prefilter fitted, I am now back to 0 on the outlet, but this is just a typical example how some of us over look the basics and go chasing rainbows, when really it is down to basic maintenance, a change of di resin is now on the calendar every 8 weeks to remind me that it has to be done, a long with the inline tds meter for quick visual checks. Total cost of this over look is probably around £100 in loss of corals, stupid rookie mistake that really I should of known better, so remember to check them basic things regular! Pump cleaning. Filter cleaning. Skimmer clean. (not just the top!) Change the carbon/phos remover, when scheduled (not the well it will be alright until next week thing we all do from time to time!) Clean the crud off the heater element, dont let it build up and cause the heater to stop working, very expensive! Dont put off what can be done today until tomorrow, it can work out very expensive. Laziness is a killer.