10% water changes are factored by adding your total tank gallons plus your sump gallons - round that number up to the next whole number. Example: 75 gallon tank with about 22 gallons in the sump = 100 gallon system = 10 gallon water change per week. Do not consider water displacement in your tank from the rock & sand. Use your total tank capacity & approximate sump gallons to factor your 10% needs.
How To Perform a Water Change:
*Be sure your tank water & new saltwater are both at 1.025 salinity.
*Be sure your sump is marked at it's operating level & your return siphon holes are clear.
*Shut down/power off your skimmer.
*Shut down/power off the system main/sump pump. Note where the water level is in the sump.
*Once the system is done back feeding to the sump, siphon or pump out your 10% gallons of water from the tank itself. NOTE: The very first time I do a water change on a new system I use a 5 gal pail to measure my gallons needed for my 10% water change & when done I mark the tank glass on a back corner with a permanent marker to show where the water line is so the next week I just siphon/pump down to my mark. No pails needed anymore!
*Refill to sump with prepared saltwater (for more info on saltwater preparation see article "Water Changes"). I fill the sump about 1/2 way to the shut down water level then turn on my pump as I continue to fill. Turn off the pump & repeat if needed so the pump doesn't suck air. Finish filling to your operating mark with the pump running.
*Clean & replace the cup on your skimmer or just turn it back on.
The next week you'll follow the same steps but won't have to "pail" the water. Just slip a 1/2" ID hose onto the outflow of a Cobalt powerhead & the other end of the hose goes into a drain (some even go out a window! :c) Pump your water down to your tank mark then refill. Again, you can siphon vs pump too.
Many ask me whether to refill into the main tank or sump. I like to refill in the sump so the cooler new water mixes with warmer sump water. Once the sump is nearly full I turn on the main pump until the pump is about to suck air then I turn it off again & continue filling until the system is full to my operating level mark in the sump. For those of us with auto top off I fill the sump a hair below it's normal operating level/line. This way the auto top off finishes the fill allowing the skimmer to slowly come back up without blowing/overflowing its cup.
Tip #1: Put your pump & skimmer each on a dedicated power strip or Belkin power switch so you can just flip the switch to turn on & off your pump & skimmer. Some using multiple heaters choose to plug them into one power strip so they can be controlled by a flip of a switch. Turning off the heater(s) is safer when lowering the sump water level so that the heater(s) don't accidentally get exposed to air while running & hot which can ruin the heater. :c)
Tip #2: If you have algae, detritus, cyano etc. you can start your hose siphon & suck up any nuisance material. Do NOT siphon down into your sand bed ever! If needed only siphon the very top sand grains to remove target material like cyano & never try to reuse the sand removed.
Tip #3: I like to insert a 12 - 24" length of 1/2" rigid tubing (depending on tank depth) into my hose end to give me more hose control. It also works great to pick off or scrub stuck on materials like picking off Bubble Algae.
Tip #4: You might consider testing your new saltwater for Alk & Mg prior to use & dose if low in either element. If you don't & either are low you will lower those elements in your system which is not good.