Purple sand

[JumperAlex]

It is a 36g FOWLR, filtration is a fluval 306 canister filter rated for 70 gallons and a fluval hang on back filter rated for 50 gallons, 35-40lbs live rock and about 40lbs live sand. The only livestock in the tank are 1 1/2 inch clownfish x 2, banggai cardinal x1 and a 4 inch chocolate starfish. I do either a 10% change weekly or 20% bi-weekly if I can't get to it. Can anyone explain what the purple crystal splotches in my sand are? And what I did to get them/what I can do to fix them?

 

[bonniewoodruff]

Is it hard? Kind of looks like coraline algae. What are your parameters?

 

[JumperAlex]

It is harder then any algae I have ever dealt with but it is also my first SW tank. I'm trying to avoid having this happen in my 90g FOWLR tank(it has a far different filtration system and is not showing any signs of similar issues) Ph 8.4, Ammonia 0, Nitrite 0, Nitrate 10ppm, SG 1.026