Water changes is one way to rid nitrates about 25% a week till in acceptable range 5ppm or less then 10% week to maintain. I personally do my water change 25% every 2 weeks.
That I don't know, but I know I don't look at my duncans and zoa's after 15 days and say their not growing give them a month . most zoa's will grow a head a month(usually) duncans grow slower than zoa's.
Generally a skimmer should be at least rated for 1.5 times the size of your tank some even do skimmers rated for 2 times the size of their tank. 75gallon tank = 100-150g capacity skimmer, preferably 150g capacity skimmer.
Cant we do anything without test?
No.
You really need to test the water.
BTW its not algae. Its cyano bacteria. Very common. looking at the pics you also have some good green algae growth. That indicates high nutrients(over feeding or a maturing cycle)
Reducing nitrates and phosphates is very important to keep these things out and testing will tell you when you have too much.
Another source of food for cyano is bad water. If you are mixing your own salt make sure you are using RODI water. VERY important. some say just RO but play it safe. Looking at the corals there is a bigger problem in the tank than the cyano and algae.
You seem to have hang on back aquaclear filter and a sump. Why?
There also appears to be no other source of flow in the tank. corals need good flow.
BTW a new tank should really only be a 10% waterchange every week. it keep it stable. large water changes cause a lot of stress, usually because of the sudden change in PH and temperature.
Actually you know what i am beginner and dont know much about these stuff.in the start i only have sump with builtin skimmer but thrn someone told me that hangon aqua filter is very important it keeps you water crystal clear so thats the only reason i hang it on
No Use one other the other. the skimmer is the filter. The aquaclear is a filter and you dont need it..
At the risk over being overly honest.
That tank is not 7 months old is it?
Actually i have skimmer which comes with sump and i was reading online,many people was saying that builtin skimmer with sump dont works good so you should put one more hang on skimmer.to be honest with you i am really confused what should i do