I used to do them whenever I had time and my tank looked good, but not great. I now do them every Wednesday and I do see improvement. I think when you get into a routine your tank just does better. With this said I am just finishing up Plumbing in a Genesis Renew Auto water change system and I am going to move to this and do the water change daily 1 gallon at a time.
Currently change 20 gallons on my 125 total gallon system weekly
I change 75 gallons on my 550 Gallon System.
I do this amount because it is just easy for me to mix up this amount of water and I can do water changes, socks, and skimmer clean up in less than a hour on both tanks.
I also do feed heavy pellets three times a day on auto feeders and Heavy doses of frozen 2 to three times a day.
I also feed my corals.
Fridays I dose Pohls extra and Coral Vitalizer, Sundays I dose Acro Power, and Tuesdays I baste my corals the night before water change with Reef Roids and Phyto.
For me Nitrates above 5 and below 20 on Salifert and P04 above .05 and as high as .15 with good colors.
I changed to Instant Ocean about a year ago and started weekly water changes and my tank has never looked better.
I dose, but will be moving to CA RX soon as my dose is up to 300 mils of each per day.