The Salty Side: Saltwater Aquariums for Beginners

Does A Frag Tank Need A Skimmer?

To ascertain the requirement for a protein skimmer, you need to ask the question, ‘Does a frag tank need a skimmer, and what will a skimmer do?.’

When we think of a frag tank, we often imagine a saltwater tank with its only occupants being coral, and maybe a couple of fish.

The perception is that there may be limited dissolved organics, detritus and other waterborne undesirables for a skimmer to withdraw from the water column.

skimmer in frag tank

Frags And Their Requirements

Our main aim in a frag tank, as in any tank which contains corals, is to ensure that our corals are provided with the correct amount of nutrients, that is nitrate and phosphate.

There are three ways we add nitrate and phosphate to a tank:

  • Feeding coral specific food.  

  • Adding fish food will contribute to an increase.

  • Dosing Nitrate, read here.

  • Dosing Phosphate, read here.

It becomes a balancing act between ensuring you do not add excessive nutrients, and ensuring that you do have enough.

In an ideal world, our aim is to add just the right amount for the requirement of your corals, without having to worry about any build up.
The reality is that it can be likely to lead to a build up, specifically phosphates.

A protein skimmer will help with the control of both nitrate and phosphate.

The fact is that a deficiency in nutrients can be far more detrimental than excessive nutrients. But how do we manage the happy medium?

Nitrate Control In A Frag Tank

Nitrate is primarily introduced into a frag tank by fish consuming food which you have fed them, and converting it into ammonia – the first part of the nitrogen cycle.

The tendency to keep only a few fish in a frag tank often leads to a deficiency in nitrate. To counteract the deficiency we are often temped to feed more. The problem is that this increase will also contribute to climb in phosphate.

How do we ensure that nitrate is maintained at a level suitable for your frags?

Without raising phosphate, reef keepers often begin dosing nitrate. This is in the form of Potassium Nitrate, or Sodium Nitrate.

Most frag tanks do not suffer from high nitrate, and for that reason we don’t often need to consider excessive nitrate control.

Unfortunately a protein skimmer in a frag tank will be indiscriminate. Skimmers are very efficient at removing the potential for nitrates in your system, by removing any waterborne fish excrement or detritus.

Redfield Ratio in Saltwater Tanks

Phosphate Control In A Frag Tank

Phosphate, unlike nitrate, can build up in a frag tank if left unchecked.

Phosphate is introduced by the food you are feeding your fish and your corals on a daily basis. If not monitored correctly it is likely to exceed natural phosphate to nitrate ratios (see Redfield Ratio).

In a reef tank with many fish, establishing a robust means of phosphate export is essential in maintaining a preferred phosphate level. However, this aggressive approach in a frag tank has the potential to strip the system entirely of phosphate.

This can only be detrimental to your coral inhabitants.

For this reason, using a protein skimmer as your preferred method of control ensures that your system isn’t completely stripped of phosphate.

How Does a Skimmer Affect Nutrients?

In summary, considering the above, the likelihood is that you can expect the following:

  • Nutrients: In the absence of a protein skimmer both nitrate and phosphate will increase unchecked. They are likely increase out of balance as phosphate exceeds nitrate.

    With the addition of a skimmer nutrients will be controlled to an extent, but must be monitored.

  • Nitrates: Expect a deficiency of nitrate and plan to supplement in a balanced ratio with the phosphate level in the tank.

  • Phosphates: Expect phosphate to rise out of ratio with nitrate, but use a protein skimmer to minimise the increment.

Is a Skimmer Necessary For My Frag Tank?

A protein skimmer removes the proteins and other dissolved organics from your system. In addition it also removes waterborne coral food and fish food. 

Effectively you are enabling the skimmer to control your phosphate levels, as well as minimising your nitrate levels before the food has a chance to break down in your system.

The proof will be in the skimate.


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