How much?
Base it of what you feed the fish and how often, watch the nitrates.(I know you do)
How often?
Again based on how much and how often you feed.
Personally I look at water changes as a process with diminishing returns. Your doing 25% change to your tank adding new clean water to some polluted water in an attempt to clean it or reduce its pollution content. You can't do this too often however because you need the bacteria in the water to keep the waste in the water under control. That being said you could probably do a water change every day if you wanted and have super clean water(not recommended). I believe many do the weekly and bi-weekly water changes because it works for them and its a nice way for us who are not meticulous to keep a happy reef.(I.E. Me weekly 20%) So if you have to have some rules to follow based on your systems I can give my opinion(not professional reef advice just my personal opinion).
I have to change the order and add some to your list:
Feeding - eye of fish is roughly stomach size of fish(till you make them fat, then the size increases)
Light-1 eye size per fish
Med-2 eye size per fish or 2x daily feeding
High-anything more than above(fat fish time)
2) biomass - Measured by inch of fish per gal of water volume
Low=25%
Med=25-60%
High=60-200% <=== Those people exist and have amazing filtration and generally large tanks and sumps I think Ron Reefman is one of those guys
1) tank size
Small(anything less than 20gal) Weekly changes due to small total system volume for filtration and bacteria
Med(20gal-90gal) Bi-Weekly changes
Large(90gal-300gal) Bi-Weekly to Monthly
Reef(300gal+) Can get away with a lot depending on bio load and dosing
3) species of fish
Only matters on how big those bad boys get and or are already
4) level measurement results
Give feeding and biomass levels of measure 1,2,3 (low/med/high) add together divide by 2 to determine percent of water change. Follow tank size recommendations for water changes.
This is the best I can come up with based on the question asked. I personally as mentioned earlier do 20% weekly to both of my 150gal tanks with their 300gal total system waters (2 300gal systems) I am med/high biomass and high feeding. I do more than what I listed cuz I have a lot of corals in 1 system and the other cuz its 110% stocked by my measurements(looking to add more). I would also like to state again I'm not a pro I look on these forums because I want to learn as much as I can and some people ask questions about things I never knew could happen prior to that reading. I can only give what information I use and practice but that doesn't mean its right just means I haven't killed my fish/corals on their 1st 16mo with me. (some fish absolutely died during the move, I do have 4 original 1st fish I ever purchased though)
I hope this helps in some way shape or form and that the real reef pros here can maybe edit it if it is helpful to you.
Nice post! Thanks.
I understand that there is no hard and fast rule, but like anything challenging in life, I try to approach it with some sort of equation and tactic.
Hobbyist 1: "I do a 25% water change every 3-4 weeks"
Hobbyist 2: "I do a 10% water change once a week."
Hobbyist 3: "I do a 40% water change every 6 months..."
My point is...there needs to be some rules for new guys like us.
This is how I would approach it...building off your suggestions
I'm sure if you figure out all the variables (Tank size, Biomass, frequency of feeding, species of fish) you can come up with a rough rule (if you give each species a number ranking based on difficulty).
Lets say that rule ends up being 25% once every two weeks.
Then you can modulate this formula with weekly testing. As long as your test results fall within a given range, you continue with the calculated water change rule.
Any deviation from the testing results should yield some form of modification in wither volume or frequency.
A Nitrate level greater than "x" means that 25% water change every two weeks is now a 15% water change every week until Nitrate levels fall below "x." Then you return to 25% every two weeks.
Sorry...I may seem like I'm sucking all the fun out of this hobby, but for me this is fun. I have an excel spreadsheet for my 16 gallon bio cube, it's the way I approach these sorts of things.
