Trickle Filter

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Trickle filter bioballs.jpg
Trickle filter with bio balls as bio-media


Trickle filters are a system of aerobic biological aquarium filtration. They are also referred to as:

  • the "dry" section of a "wet and dry filter"
  • "mini reef filters" (in the 80's parlance)


They consist of a trickle plate, spray bar or sprinkler to trickle aquarium water over a chamber filled with high surface area medium. The media used are mostly plastic cubes or balls (bio balls), ceramic rings / noodles or coral rubble.

The filter medium provides surface area which, due to constant flow of a thin film of water in the presence of air, becomes a highly oxygenated environment. This becomes colonised by aerobic nitrifying baceteria which convert toxic ammonia and nitrite from metabolic waste to the (relatively) benign [[nitrate]. This process is referred to as nitrification. (See also: Nitrogen Cycle)

The trickle filter may be located:

  • in a sump beneath the aquarium, gravity fed and using a pump to return water to the aquarium, or
  • above the aquarium, fed by water pumped from the aquarium and draining back into the display by gravity.



There is no more effective system for the process of converting ammonia to nitrite to nitrate. The development of the trickle filter was a breakthrough in successful marine fish keeping in the 1970's and 80's. There are still applications where a trickle filter may be considered appropriate, such as fish-only systems and large scale commercial facilities, where it's ability to rapidly convert harmful waste to nitrate, and also its ability to rapidly respond to changes in bio-load make it very effective at what it is designed to do.


Elevated Nitrate

It's success however, is also the reason that the trickle filter is no longer commonly regarded as appropriate for use in the modern reef aquarium that houses many corals and other more sensitive organisms.

Whilst the highly aerated environment of the trickle filter rapidly converts nitrogenous waste to nitrate (nitrification), it does not provide any scope for the housing of anaerobic bacteria that are able to further reduce nitrate to nitrogen, which is otherwise then able to migrate out of the aquarium by off-gassing. The bacterial reduction of nitrate to gaseous nitrogen is referred to as denitrification. The result is that aquaria running trickle filters will normally be observed to have an elevated level of nitrate in the water column. This has led some to refer to trickle filters as "Nitrate factories".

Whilst at moderate levels (say < 30ppm) nitrate is not harmful to most fish, it is detrimental to many corals and organisms which are intolerant of elevated levels of nitrate >10ppm.

This is in contrast to other "natural" filter systems such as live rock and deep sand beds (DSB) which are able to accommodate anaerobic bacteria in close proximity to aerobic zones. In such systems, aquarium water diffuses through aerobic bacterial zones, delivering nitrate directly as a food source to the anaerobic bacterial zones. As a result, many established systems with sufficient live sand and live rock may well have an undetectable level of nitrate in the water column.

Further, the presence of a trickle filter in conjunction to a deep sand bed may well impede the denitrifying capability of the DSB, since any ammonia and nitrite present will be converted to nitrate before it has a chance to get anywhere near the sand bed.

Trapping Particulate Matter

Another reason that trickle filters have a tendency to produce nitrate is that they can provide a mechanical catch point that can trap food and other particulate matter, including beneficial micro-fauna (eg 'pods), especially when placed immediately after the overflow. If caught in a filter, this organic material will decay, and will manifest itself as dissolved organic and inorganic waste (including nitrate and phosphate).

In the absence of a trickle filter (or other mechanical filter) this organic material is able to be dealt with by:

  • particulates suspended in the water column can be removed by a protein skimmer, or
  • particulates allowed to settle on a live sandbed or in a refugium can be utilised by other organisms,

Power Outages

Another concern with trickle filters is that if the power goes out for a long period of time, and the trickle filter is allowed to dry out, the bacteria can all die. The dead bacteria, along with the reduced filtering bacterial populations may then cause an ammonia / nitrite spike in the system.

Current View

It is possible, of course, to use trickle filters in a reef system with success. They have been used for many years, and there many aquarist's who still swear by them. However the generally held view is that trickle filters are not suitable for reef systems. At best, they are unnecessary, and at worst their propensity for elevated nitrate levels is detrimental.

If trickle filters are used, efforts must be made to ensure that nitrate levels are not elevated. This can be done by:

With many reef systems, the benefits that trickle filters provide are considered minimal or even negligible. Whilst not necessarily a negative inclusion, neither do they provide any substantial benefit, especially in a system utilising sufficient live rock and live sand. There is generally more than enough surface area on the substrate of the tank for the nitrifying bacteria to live.

For many reefers, a trickle filter is a piece of extraneous equipment that was sold to them when they first purchased their aquarium system, often at substantial cost, and which is no longer needed. There are many veteran aquarists who will admit to having a bucket of bio balls in the back shed.

On the other hand, some reefers love them.

Either way, before purchasing one, or removing one from an established system, it is important to understand the pros and cons listed here, and what other forms of filtration are available. It really is a matter of preference, and selection of the right piece of equipment to achieve the aims of the aquarist.



  • Rapid conversion of nitrogenous waste to nitrate
  • Rapid response to changing bioload
  • Assists in maintaining high oxygenation levels in the aquarium


  • Likely to result in elevated nitrate levels
  • Expensive
  • Inhibits natural denitrification in sand beds
  • Source of entrapment for organic / particulate waste (detritus)
  • Loss of filtration capacity due to power outage
  • Considered by many to be unnecessary