Tank is 10mths in, battling what i think is dinos for first time. Did paper towel test, didn't clump, thought it was diatoms. Got worse after two weekly water changes. Tested for silicates ok. But in tank noticed a couple of strands with bubbles trapped. Also I've had 0 phos and 0 nitrate since cycling. Wasn't too worried about it but I guess this is the result. Also my urchin had a dead crab body on his back last week. Watched brs vid on dinos. Been scraping the rocks 1/2 times a week (filter roller satisfyingly kicks in a lot when I do). Started dosing waste away weekly. Struggle to get it off sand though. Not done a water change in 3 weeks. Its smothered a newish rasta zoa. Its growing in/around my other zoas so am worried. Other corals are fine. Trying to overfeed but not sure how much? A cube of frozen mysis every couple of days? Am I doing the right things? 500litre tank. Params in range and v stable (salt 35, ph8.3, kh9.2, calc 450) - apart from 0 phos 0 nitrate - also stable for months). Pics of sand, rock, and a partially cleaned bit of rock below.