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Coralline is strange to me. I have never in 10 years been able to get coralline encrusted rocks that I have seen in other tanks and I do not know why. Some of my observances in my own personal experience are. The darker the tank the more Coralline I had and the faster it spread. Also when I dose 2-part on a daily basis I had coralline, but only on the glass. I have tried all of the snake oils like purple up, aragamilk, etc and never saw a difference. Scraping is supposed to seed the tank, but I'm not so sure about that so much because with all of the scraping I did, everything should have been covered.
I've tried Purple Up (actually got 2 bottles and used them both up). While it did promote some minor growth with the first bottle in the early stages (when my tank had just done cycling) it didn't help all that much after that. I saw no increase at all with the second bottle. As Ryan stated, it's simply a CA buffer and it fixed my params enough to get it started... but didn't make it grow any faster.
I also scrape mine off the glass pretty often and that does seem to help it spread a bit. Nothing drastic though.
I think keeping your CA in the upper end of the acceptable range is the key to making it grow faster. People with CA reactors and kalk reactors tend to have the purple covered tanks. I believe lighting has a lot to do with it also. For instance, I have some corraline growing inside my skimmer body... but it only grows in one area that gets exposed to my fuge light. I've even cleaned it all off once, then spun the skimmer clockwise 45 degrees, and a few weeks later I had another growth in the section that faced the light.
Keep your lights low for a few weeks...if your water parameters are good, you will get more coralline then you need or want...same parameters as you would need for good coral growth
I agree with others,scraping in high flow will help "seed" the tank.Also I found adding kalk weekly or bi weekly also helps,Ive also did doses of iron and that helps.Low lighting is also a factor.I had a 90 gal several yrs back and liked the actinic vho supplement and the coraline exploded grew everywhere,I swear if the fish didnt move fast enough it would of grown on them too.
My tank cycled in December, and i am getting purple spotty all over the tank. even on the MP40. I have never had to add anything to my tanks for coraline. My daughters tank is getting the back of the tank covered, that has been up since June.
When I used oceanic salt my tank way covered. Since I made the switch to esv salt it all went away. About 3 weeks ago I made the switch back to oceanic now it starting to cover everything. So beats me what the difference is. cal mag alk always in the same range