So far we know you have a 72 gallon tank with 3 fish and you do about a 10% per month PWC.
So now for the questons:
What type of filtration are you using?
20 Gallon Sump
What is the substrate?
LS 100lbs
How much LR do you have?
67 LBS
How much water movement do you have?
2 Kori (3)
What type of lights and wattage?
Nova Extrem Pro
How long are the lights on each day?
5 Hours
What type of water do you use for top off (RODI/Tap)?
RODI
What type of water do you use for the PWC?
RODI/Reef Crystals
What do you feed the fish?
Flak food/ Frozen
How often do you feed and what do you feed?
every other day
What does your clean up crew consist of?
40 Hermit crabs
Are you using any supplements? If so, what, what dosage, how often?
none
What has changed in the past 6 months?
Nothing
What are the water readings for
Temp 78
SG 2.25
pH 7.5
Ammonia 0
Nitrite 0
Nitrate 5-10
Phosphate
Calcium
Alk
Magnesium
Please give actual numbers for those items you have a test kit for.
Then we can try to help