Urgent help

[Sowmithrichandr]

I have a 15 gallon reef tank recently established with the help of the guys in this forum. I started it and running from 2 months. 10days back i got a Favia coral and a unknown small red coral. I posted the images of the red coral but no one identified it. But it was happy. 4days back my cooling fan fell into the water and lot of dust entered the water stream. I cleaned the dust immediatly and did huge water changes upto everything is crisp and stable. When doing changes i noticed that favia coral had some white slime covered the bottom line polyps around 5to7 polyps are covered and it is slimy so i took the coral out and cleaned the slime. The polyps under slime turned white and i think they are dead. I thought that when are extracting the corals they broke some of them so the damaged ones are dying. Its is in a medium flow on aragonite sand bed and i regularly check that none of the polyps are coverd with sand. And i regulary check the water parameters also. I use korall fluid it will open all the tenticles at night and i thinks it is happy. But today i saw another polyp is covered with the white. I am very much worried.

Tank parameter
Salinity 35ppt
Ammonia 0
Nitrite 0
Nitrate 5
Calcium 420
Ph8.1
15 galllon with hob filter having carbon phosban and slicatex
Powerhead sicce voyager 2 800gph
Led lighting 1:1 42 watt
7kg of live rock
One inch aragonite
One favia coral
One unknown coral and a dottyback 1.5inch
I feed only once a day. I only korall fluid every two days according two the dosage give on the bottle. One drop.
I change 20percent of water every week and chamge the filter floss.

 

[ThorEffex]

When I touch or stress my favia coral it slimes up pretty good but it's always fine. but the slime is not white it is usually clear. I use a turkey baster to rinse of things in my tank with a blast of water to reduce stress of removing it to clean it. Just remember that stability is the key and in small tanks like a 15 gallon it's a lot more work, I had a 5 gallon and I had enough of amount of work and the lack of automation.