Filter Floss: When to Replace It for Optimal Aquarium Health

A common question among new aquarium owners is, “How often should I replace my filter floss?”

And one of the most common answers you’ll see on popular reef keeping forums is “every 2 to 3 days”.

There are two slight problems with this rote piece of advice though:

  1. It could cost you extra money. And,
  2. It could starve your tank.

Every tank is unique, including yours. Just because changing the filter floss every 3 days works best on another person’s tank doesn’t mean it will work best for yours.

How often you change your filter floss will depend on your bio-load and the maturity of your tank.

filter floss cut into strips
Filter floss cut into 1 inch strips for placement between sump baffles.

Consider a new tank that is less than three months old…

Changing the filter floss too often could deprive the bacteria and other microscopic critters of the food they need to develop. This may result in your tank taking longer to mature than usual.

Likewise, changing filter floss too often in a established, mature tank could strip the water of too many nutrients causing the coral to starve.

So where does the common ‘2 to 3 days’ come into play?

As a starting point.

Filter floss isn’t needed until you reach your desired nutrient levels. Phosphate of 0.03 ppm and nitrate of 1 to 5 ppm are most typical.

Once your tank is at your desired nutrient levels begin using filter floss.

For the next 4 to 6 weeks replace it every three days.

Be sure to test your phosphates and nitrates at least twice a week during this period.

Then…

Let Your Tank ‘Tell’ You When to Change the Filter Floss

If your nutrients continue to rise during the test period switch to replacing it every 2 days – and consider adding additional filtration such as a skimmer and/or granular activated carbon if you don’t already use them.

If your nutrients hold steady or drop closer to zero during the test period, try replacing the filter floss every five days.

Keep testing for 4 to 6 weeks.

If the nutrients continue to hold stable or drop, bump the replacement schedule out to once a week.

Keep testing. And keep bumping the schedule out.

When your nutrients start to rise a little, stop and switch back to the previous schedule. Congratulations! You’ve found your filter floss replacement ‘sweet spot’.

For example, if they were stable when you replaced the floss every 7 days (once a week) but started to rise when you replaced it every 9 days, stop. 7 days is your replacement ‘sweet spot’.

Changing it more often than 7 days would just be wasting money. And waiting longer than 7 days will cause your nutrient levels to rise.

What if your able to go for several weeks without replacing the floss and still see no rise in your nutrient levels?

Well then you may not need to use filter floss at all. It would indicate the other biological and/or mechanical filtration in your system is more than enough to keep your nutrient levels in check.