TRANSFORMING BASE SUBSTANCES INTO GOLD

Water Treatment for Aquariums and Fisheries – An Overview

by Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM

 

Introduction:

Water treatment for aquariums and fisheries is an interesting branch of my favourite applied area of water treatment. Fish are beautiful to look at and they reproduce quickly making fisheries a sustainable source of good protein.

Aquarium water treatment involves making tap/sea water safe for the fish/plants, establishing the nitrogen cycle to remove ammonia/nitrites, and maintaining water quality through regular changes and filtration. Key steps include using water conditioners to remove chlorine, implementing a multi-stage filtration system (mechanical, chemical, and biological) to process waste water, disinfecting the water, and performing partial water changes to remove nitrates/TDS. Monitoring water parameters like pH, ammonia, and nitrate/nitrites is also essential for a healthy environment.

Fisheries/Aquaculture treatment uses large industrial systems like mechanical filters and biofilters for both incoming and outgoing water to handle high volumes and waste. 

Aquaristics is the practice of keeping and maintaining aquariums, including the care of the fish, plants, and other aquatic life within them.

 

Strategy and Risk

Water treatment for aquariums and fisheries should follow a specific strategy and have a risk assessment/management plan.

The strategy should abide by sustainability principles and adhere to nature based solutions to minimize environmental impact.

Risk assessments and management plans help reduce the risk to the health of fish, humans and the environment. For example, Lion fish are wrecking havoc on indigenous fish species in the Mediterranean because by their very spiky nature, they have no natural predators.

 

Basic Aims

Aquarium water treatment basically aims to:

1. Filter out particles of excess food or waste or dust
2. Remove ammonia/nitrites by converting them into nitrates or even completing the N cycle by converting the nitrates into N2 gas. This is accomplished in the biofilter where beneficial nitrifying bacteria colonize some kind of media with high surface area. Denitrifying filters convert the nitrates to N2 gas. 
3. Trickle filters also remove the CO2 from respiration which affects the pH
4.  Protein skimmers remove DOC or Organic Matter by creating air bubbles in a column and collecting the foam which contains the DOC. Used mostly in marine aquariums. 
5. Active Carbon filters in Bio-filtration systems remove any medications/metabolites/organic molecules that may have accumulated in the water. Obviously they will also remove the chlorine from the potable water.
6. UV lamps kill pathogenic bacteria without the use of chemicals. 
7. Air bubbles Oxygenate the water          
8. Aerobic denitrification or co-respiration is the simultaneous use of both oxygen (O2) and nitrate (NO3−) as oxidizing agents, performed by various genera of microorganisms knowns as ADB’s. This process differs from anaerobic denitrification not only in its insensitivity to the presence of oxygen, but also in that it has a higher potential to create the harmful by-product nitrous oxide.
9. Nitrate, acting as an oxidant, is therefore reduced in a succession of four reactions performed by the enzymes nitrate, nitrite, nitric-oxide, and nitrous oxide reductases. The pathway ultimately yields reduced molecular nitrogen (N2), as well as, when the reaction does not reach completion, the intermediate species nitrous oxide (N2O) – which is undesirable. High concentrations of N₂O (nitrous oxide) can be dangerous and toxic for fish. Nitrous oxide is part of the nitrogen cycle, but its accumulation, particularly in the form of its precursor, nitrite (NO₂⁻), can be lethal to fish even at low levels. 
10. To adjust hardness or alkalinity: add hard or alkaline salts or use RO or add distilled water.
11. Natural algae/plants help in filtering out nitrates  
12. Ferric Dosing for Removal of P from the waste water
13. Heat Exchanger/heater for tropical fish in case the water is too cold or water cooler in case the water is too hot for some cold water fish species.
14. Zeolites are widely used in aquaristics primarily for the short-term removal of toxic ammonia and heavy metals in freshwater aquariums. They act as a chemical filter medium through a process of ion exchange, where harmless ions in the zeolite’s structure are swapped for harmful ammonium ions (NH4+)/heavy metals in the water.
15. Sometimes Mixed Bed Resin or RO is used to reduce the TDS.
16. An ATO (Automatic Top-Off) system is an automated device for aquariums that replenishes water lost to evaporation/blowdown. By consistently maintaining a stable water level, it prevents fluctuations in salinity and other water parameters, which is crucial for the health of fish and corals. These systems typically use a sensor in the tank, a pump in a freshwater reservoir, and a controller to automatically add water when the level drops.
17. Dosing pumps automate the addition of precise, measured amounts of liquid supplements to aquariums, which helps maintain stable water parameters and reduce manual work. They are especially important for reef tanks to consistently provide essentials like calcium, alkalinity, and trace elements that corals consume, preventing fluctuations that are detrimental to the tank’s ecosystem. They can also be used for adding coral foods, liquid fertilizers, or performing automated tasks like water changes.
18. Ozone is dosed into aquarium water specifically to kill pathogens such as harmful bacteria, viruses, fungi, and parasites. It is a powerful oxidizing agent widely used in commercial aquaculture and by advanced hobbyists to maintain pristine water quality and reduce disease outbreaks.
19. Light fixtures are attached to indoors aquariums where there is no access to day light to promote the growth of algae/corals.

Note: A downflow hanging sponge (DHS) reactor is an innovative aerobic wastewater treatment system that uses hanging polyurethane sponges to provide a large surface area for biofilm growth.

Large Aquarium Waste Water Treatment

In 2025, I consulted for a large seawater aquarium project on Gibraltar. The main consultant proposed a physico-chemical-biological treatment system for the aquarium but the Environment Agency asked them for a Strategy document. They asked the designer what is the strategy behind water treatment for the aquarium. They were also worried about the waste water that is discharged to foul sewer from the backwash of the pressure sand filters.

The fish microbiome differs significantly from the human microbiome primarily in its dominant bacterial composition, the influence of the external environment, and the physiological conditions of the gut. Pathogenic bacteria that infect fish, and which can be dangerous to humans, include Mycobacterium marinum, Vibrio spp., and Streptococcus iniae. These can cause serious illnesses, especially in immunocompromised individuals or when they enter the body through wounds.

So I proposed an MBR system to treat the pressure filter backwash water through which the risk of fish disease infections is reduced and the waste water can be recycled.

Monitoring Aquarium Water Quality:

1. Monitoring Ammonia, Nitrites and Nitrates is very important
2. Monitoring pH is important. High or low pH can stress the Fish
3. Monitoring hardness and alkalinity is important as low alkalinity can lead to unstable pH. Some reef tanks need Calcium for their corals. 
4. Last but not least the Fish need to breathe hence DO monitoring is very important
5. You want to watch that BOD coz it will consume DO
6. Monitoring temperature as some fish species, like tropical fish, love warmer water others the opposite.
7. Monitor Salinity or TDS. Note that for marine aquariums, refractometers are generally considered better and more accurate for measuring salinity than standard TDS instruments. TDS meters have specific limitations in this application.
8. Monitor ORP which would be around +300 mV for normal DO levels and when dosing Ozone never exceed +350 mV.

Special Requirements Per Species:

Special aquarium needs for an octopus include a tightly-fitting, escape-proof lid, a spacious tank (minimum 50 gallons for small species) with live rock and a sand bed for hiding, efficient filtration (like a wet-dry filter and protein skimmer), and careful water parameter maintenance. You must never use copper-based treatments, and the tank must be fully cycled before introducing the octopus. Feeding should include live food, and only one octopus per tank is possible due to their solitary and predatory nature.

Pristine conditions are vital for seahorses. Ammonia and nitrite must be undetectable (0 ppm), and nitrate kept below 20 ppm. A robust filtration system, including a protein skimmer, is essential.

 

Key suppliers in the EU/UK

Aquaforest: A Polish company that provides a wide range of products and lab services for aquaristics, with a strong focus on marine and reef tanks. They offer a variety of water supplements and analytical tools to maintain a healthy aquarium environment.

RP Aquatics: Based in the UK, this company offers a broad selection of water treatment products and has over 15 years of experience in the aquatics and Koi industry. They provide both standard and innovative solutions for aquatic filtration.

Royal Exclusiv: A German company that manufactures high-quality filtration and pump systems for large aquariums, including their signature Dreambox filter systems. Their products are known for their quality and customizability.

Puretech Water Systems: While based in the UK, this company specializes in large-scale commercial and aquarium water purification solutions, such as reverse osmosis systems. They have experience supplying and maintaining systems for major aquariums in the UK.

e-Marine Aquatics: Specialist marine aquatics business based in Wickford, Essex UK.

Swiss Aqua Shop

Leading World Class Aquariums and Institutions

• Okinawa Churaumi Aquarium: Located in Japan, it is frequently cited as one of the world’s best and largest aquariums, known for its massive “Kuroshio Sea” tank and focus on large marine life, like whale sharks and manta rays.
• Numazu Deep Sea Aquarium: This aquarium is notable for specializing in deep-sea creatures, a niche within the aquarium world, according to the Japan National Tourism Organization.
• S.E.A. Aquarium: Located in Singapore, this is one of the world’s largest aquariums, holding a vast number of marine species and significant water volume, as noted by Wikipedia.

References:

1. https://www.researchgate.net/publication/317414390_A_nitrogen_removal_system_to_limit_water_exchange_for_recirculating_freshwater_aquarium_using_DHS-USB_reactor

2. https://www.hannainstruments.co.uk/blog/post/62-what-are-the-key-aquarium-parameters-for-freshwater-tanks

 

3. https://www.faunamarin.de/en/knowledge-base/

 

Compiled by Rami E. Kremesti M.Sc., CSci, CEnv, CWEM

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About the Author

Rami Elias Kremesti grew up in Beirut, Lebanon where he used to go fishing and snorkelling in the Mediterranean – which can be highly polluted from sewage runoff in some areas near the major cities. During his teenage years, his love for chemistry grew and he got his bachelors and masters degrees both in chemistry. After a failed stint at a boring PhD, he decided to work in practical science and established a career in water treatment. He currently lives in the UK after having worked around the world on power station projects with ALSTOM. For him, the fact that fish can breathe from 10 ppm DO water is a miracle of nature.