[thedoc (OP)]

Ruth  asked the Naked Scientists:
   
Hi,

My name is Ruth am trying to help my son with his grade 6 science project, he wants to see if it's possible to reuse the water in the fish tank after cleaning it, what I mean is after cleaning the tank you purify the water and reuse it again in the fish tank.

We have come up with a manual way of purifying the water by putting it through the rocks and sand, but after that has been done we are not sure if the water will be good enough for the fish, if not so what can we use to kill the bacteria that we can produce from home. I.e is bleach good for fish?

And if we only recycle like 20% of the water from the tank and the rest of the water just use without purifying will that have any negative effect on fish.

Looking forward to your response.

Ruth Pila
Mom of grade 6 learner

What do you think?

[Bored chemist]

"I.e is bleach good for fish?"
No
According to this
http://pmep.cce.cornell.edu/profiles/miscpesticides/methylchloride-xanthangum/hypochlorite/hypochl_prf_0286.html
less than a part in a million of bleach in the water will kill the fish.


Even the traces of chlorine in drinking water are bad for fish.

[CliffordK]

Plants are great at "recycling" water.  For example, Beta fish are often kept in a tank with plants growing.

I would encourage testing your water's pH.  It is quite possible you would get a slow buildup of ammonia in the water.  You might be able to counteract it to some extent by adding acid.

However, perhaps an approach would be a two-tank system. 

Empty your fish tank into a secondary algae tank (raw, unfiltered).  Let the algae grow in good sunlight for a few weeks.  Filter, test, then put it back into the fish tank.

[Don_1]

The main thing you need to get rid of are nitrates. Unless your fish are displaying signs of poor health, such as fin rot or any fungal growth, it is unlikely that there are any harmful bacteria in the tank. That's not to say there won't be bacteria in the tank. But some bacteria are allies in keeping your tank healthy.

As BC has said, chlorine is a no no. Even the low concentration in tap water can strip mucus from fishes gills, which can result in them drowning.

After filtering out the visible rubbish in the water, use a carbon** filter in the same way you have devised a gravel filter. This will deal with nitrates and ammonia. Re using the water is a good idea, so long as you get it clean. Fresh water from the tap, even after being left to stand for 24hrs for the chlorides to evaporate off, or treating with a proprietory 'tap safe', will still contain nutrients which will promote the growth of algae.

**You can buy charcoal (or carbon impregnated sponge) from any good aquarist. Don't use the charcoal sold for BBQ use, it isn't pure enough.

[CliffordK]

If you have chlorinated water...  Find a friend that has access to country well water.  Or...  perhaps find some good clean river water.  Or... water from your rain gutters.

I'd experiment a bit with some cheap goldfish before you start re-using the water in your main tank.

[Don_1]

If you have chlorinated water...  Find a friend that has access to country well water.  Or...  perhaps find some good clean river water.  Or... water from your rain gutters.

Do be careful not to introduce parasites.

I'd experiment a bit with some cheap goldfish before you start re-using the water in your main tank.

This I cannot condone and I am frankly astonished at the suggestion.

[Don_1]

Rather than periodic treatment or changing of the water, it is by far better to have an ongoing system to keep the water healthy.

Plants can certainly help in this, as can daphnia and tubifix. The daphnia will swim around consuming algea and the tubifix will get into the gravel bed, where they will consume decaying material. Both also provide good live food for your fish. Filtration systems can be bought from your aquarist which will usually employ a carbon impregnated sponge for the in-tank systems, though larger and more effective external systems may use multi chamber gravel and carbon filters. Both have the added bonus of helping to aerate the water.

[Mazurka]

As Don says, the many problem is nitrogen compounds.  It is however important to remember that aquariums are not a closed system because the fish are fed.  As a consequence it is very difficult to try and establish an equlibrium. Fishtanks also tend to be stocked at a much higher density than the fish would encounter in a natural system. This makes it harder to establish a self sustainign natural cycle.  (Many garden ponds have difficulties with the likes of duck weed / algae because the nitrate levels are too high.)
 
Fish excrete ammonia, which is broken down into nitrites by nitrosomonas bacteria and then to nitrate by nitrospira bacteria. Some nitrate is then taken up by living plants & algae in the tank - however frequently the avaliabillity of other compounds limits plant growth (thus the amount of nitrate that plants can remove).

It is very difficult (in cold freshwater tanks) to keep nitrate levels under control without periodically removing water.  Although it can be acheived with certain seawater set ups where (anaerobic) dinitrifying bacteria can break nitrate down into nitrogen gas, however such setups nearly always include a "protein skimmer" to remove other undesirable contaminants.

DO NOT USE BLEACH!  DO NOT USE SOAP! If I need to sterilise a hospital tank/ nets other equipment etc I use peroxide based bleach and then cambden solution and then rinse thoroghly with tap water 5 or 6 times. I also keep a bucket soley for water changes so that there is no chance of soap residue etc contaminating the tank.

It may be possible to set up a series of tanks/ processes that could treat aquarium water (such as carbon filtration, sand beds etc.) but this would require regular testing to ensure the water is clean (and nitrate free).  I would expect this would require a number of steps like filtration, adsorbation, nitrification of ammonia and dinitrification and reoxegenation.  I also note that carbon filter media has to be renewed frequently to prevent it releasing contaminants back in to the system when its adsorbative capacity has been reached.

Perhaps as part of the project you could look at the aquarium test kits that are avaliable, as these indicate the compounds or factors (such as pH and hardness) that are important in an aquarium?

I would suggest that the most best solution is to water houseplants with old fish water (they love it) unless you add a little salt to your tank (which is the old school aquarists cure all) and put fresh water in.

Ideally from a water butt or well (whcih may need filtering but will be nice and "soft" and chlorine free) but from a tap  is fine if you use  a product such as "tap safe" or water conditioner, which lock up the trace heavy metals and (more importantly) chlorine in tap water (i'm not sure exactly how).  A freind of mine swears by simply leaving a bucket of water standing in the sunshine to remove chlorine. 

I would not recomend river water as it may contain (low levels) of pollutants such as nitrate/ phosphate from agricultrual run off or sewage treatment which would defeat the purpose of a water change...

[richardtj]

The issue with the waste water can be divided into:
1) Solid Waste (ie the fish poo)
2) Chemical Wastes and decomposition.

The solid wastes can be removed easily with filter material (typically a wool in commercial filters), but the use of granular materials works fine.
The chemical wastes typically consist of methane (from anerobic decomposition), which has yet to be turned into nitrates.  These are converted via bacteria, which thrive in such materials as zeolite.  However, they have to be grown first- ie you need a colony of them.  Activated Charcoal also removes these checmical compounds from the waste water and once spent can be re-activated by heating them in an oven for a few hours.

Commercial sites use reed beds to purify waste waters, with effluent being drained through them.

[Mazurka]

Depending on how a tank is filtered, its actually quite difficult to establish anaerobic conditions in an aquarium.  A healthy tank will be well oxygenated and the water will pass through the filter 3 or4  times an hour  (the filter in my 140l tank has a flow rate of around 600l an hour)

To create anaerobic conbditions in a tank you need a really thick (At least 4cm) layer of gravel/ sand with a poor water flow through it (so no undergravel filter).  However if there is a poor water flow it is difficult to get the bugs to work for you...  There is a system called a coil denitrifyier where tank water is passed very slowly through a long narrow pipe. Normal aerobic bacteria live at the front end and anerobic bateria live futher along.  I understand it can be very effective, but does require water to be pumped from the tank and are very tricky to set up. 

Unless there is a lot of organic material (such as decaying wood/ plant matter in the anerobic zone) or the gravel substrate has not been disturbed for years  you are more likely to get hydrogen sulphide than methane from anaerobic bugs in a fish tank.  H₂S is toxic to fish.   CH₄ less so.  Compounding this is the solubillity of the two gasses - H₂S is 4g per litre and CH₄ is 22mg per litre - so methane will tend to bubble (and be released at the surface) and H₂S dissolve thus be available for the fish to take up.