Rocks or fil­ters?

While that might not sound a fair com­par­i­son, rocks and fil­ters in the tank can do pretty much the same thing – one per­haps bet­ter than the other!

Practical Fishkeeping (UK) - - Marine Guide -

Fish pro­duce waste, just as we do, in the form of urine and fae­ces. They also ex­crete waste into the wa­ter di­rectly out of their gills. in the closed en­vi­ron­ment of an aquar­ium, that waste has nowhere to go, so it builds up to toxic lev­els, poi­son­ing the in­hab­i­tants.

The main type of aquar­ium fil­tra­tion to deal with waste is bi­o­log­i­cal fil­tra­tion. This method utilises bac­te­ria that use fish waste as a food source, turn­ing it into some­thing less harm­ful.

in the first in­stance, fish pro­duce am­mo­nia. un­eaten food and other de­com­pos­ing mat­ter also con­tribute to the build-up of am­mo­nia. And am­mo­nia is deadly, even at the low­est lev­els.

Bi­o­log­i­cal fil­ters act as a home for bac­te­ria that con­vert this lethal am­mo­nia into lessharm­ful ni­trite, and even­tu­ally into ni­trate.

Th­ese bac­te­ria are re­ally slow to de­velop. it can take weeks or months to de­velop a colony of them suf­fi­ciently large to deal with the waste in even a mod­er­ate-sized tank. so, the bac­te­ria need to be in place and cop­ing be­fore any live­stock is added to the tank. This is done through ei­ther the ad­di­tion of live rock, or through the mat­u­ra­tion of bi­o­log­i­cal fil­ters, such as ex­ter­nal can­is­ter or sump fil­ters.

one an­noy­ing off­shoot of bi­o­log­i­cal fil­tra­tion is that it can only con­vert waste so far – usu­ally to ni­trate – and this chem­i­cal still needs to be con­trolled through reg­u­lar wa­ter changes.

Live rock

Live rock is the ‘go to’ fil­ter medium for the mod­ern ma­rine tank. Live rock is formed from old, dead coral skele­tons, and nat­u­rally oc­curs abun­dantly around reefs. here, it be­haves as a bi­o­log­i­cal fil­ter for the sea, hous­ing the nec­es­sary bac­te­ria re­quired to con­vert wastes.

By har­vest­ing this rock, aquar­ists can bring ‘fil­ters’ straight from the sea and into their tanks. While ex­pen­sive – even cheap live rock will set you back dou­ble fig­ures per kg – it’s con­sid­ered by far the best way to main­tain wa­ter qual­ity in a ma­rine tank.

As a bonus, live rock goes above and be­yond the ac­tiv­ity of stan­dard bi­o­log­i­cal fil­tra­tion. Be­cause the rock is so por­ous, the bac­te­ria pen­e­trate deep in­side it, mean­ing ad­di­tional types of bac­te­ria can oc­cur. Among th­ese are bac­te­ria who will even use ni­trate as a food source, con­vert­ing it into harm­less ni­tro­gen gas.

As an ex­tra bonus, live rock also comes with a bounty of other life forms cov­er­ing it! Along­side colour­ful al­gae (es­pe­cially the gor­geous, pur­ple en­crust­ing types), live rock can come with tiny polyps, corals, shrimps, crabs, sponges and more.

un­for­tu­nately, that also means you some­times have a nui­sance crea­ture turn up as well, such as a preda­tory Man­tis shrimp, or a coral-eat­ing worm but, on the whole, most live rock is safe.

When buy­ing live rock, make sure you only buy ‘cured’ live rock. Af­ter col­lec­tion and tran­sit, live rock turns foul for a short while and re­quires soak­ing and mat­u­ra­tion to flush out any nas­ties within it – this is the cur­ing process. While cheap, un­cured live rock is avail­able, if you add this to your tank it will al­most cer­tainly cause ex­treme wa­ter qual­ity is­sues.

Al­ways ask if it’s cured Be­fore you buy!

Bi­o­log­i­cal can­is­ter fil­tra­tion

You’ll be fa­mil­iar with bi­o­log­i­cal fil­ters if you’ve ever kept fresh­wa­ter fish. They’re the can­is­ters that sit in­side or out­side an aquar­ium, con­tain dif­fer­ent types of me­dia, and need reg­u­lar clean­ing and main­te­nance.

in ma­rine tanks, can­is­ter fil­ters have

largely been made re­dun­dant by su­per-ef­fi­cient live rock, but some peo­ple do still like to use them – par­tic­u­larly those with fish-only sys­tems.

The main draw­back with a can­is­ter-type fil­ter in a ma­rine tank is that it doesn’t have the abil­ity of live rock to con­vert ni­trates into ni­tro­gen (un­less ex­pen­sive sup­ple­men­tary fil­ters are added). This means it’s more dif­fi­cult to con­trol ni­trates, and will re­quire very fre­quent wa­ter changes to di­lute the ni­trates back down.

Can­is­ter fil­ters can also be­come dirty, so the me­dia in­side them be­comes smoth­ered and loses ef­fi­ciency. In the event of a fail­ure of the pump, the bac­te­ria in­side may starve or suf­fo­cate, lead­ing to the fil­ter ‘crash­ing’ and be­ing un­able to con­vert toxic wastes.

Bi­o­log­i­cal fil­ters also need to be cy­cled (see be­low) be­fore fish can be added to the tank. This is time-con­sum­ing and frus­trat­ing in a way that live rock just isn’t.

What is cy­cling?

Cy­cling is the act of tak­ing a fil­ter with­out any bac­te­ria and colonis­ing it with enough bac­te­ria to cope with the waste that will even­tu­ally be pro­duced by fish.

The main way of cy­cling a fil­ter in­volves adding liq­uid am­mo­nia to sim­u­late fish waste over a long pe­riod, pro­vid­ing the bac­te­ria with a food source to grow on. Be­cause the tank will be highly toxic with am­mo­nia through­out this pe­riod, fish and corals can­not be added.

While cy­cling is a long-winded process, adding bi­o­log­i­cal sup­ple­ments can help. Th­ese liq­uids and tablets con­tain colonies of live bac­te­ria that set­tle in the fil­ter. Some claim to be able to ma­ture a tank in­stan­ta­neously, but it’s best to take such claims with a pinch of salt.

Dur­ing the cy­cling pe­riod, test the wa­ter reg­u­larly to as­sess how much pol­lu­tion is be­ing con­verted, and there­fore how the bac­te­ria are de­vel­op­ing. There are help­ful cal­cu­la­tors on­line where you can up­load your test re­sults. They’ll then tell you how much more am­mo­nia to add un­til your tank is fi­nally ‘ma­ture’ and safe for fish.

Left: A typ­i­cal can­is­ter fil­ter.

Above: Bi­o­log­i­cal sup­ple­ments kick start fil­tra­tion.

Above: Live rock brings a reef tank to full glory.

Be­low: Fresh live rock is rich with life and colour.

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