01/06/2026
As fishkeepers, we know our source water - what we use to replace tank water when we do water changes - is incredibly important. So last night I had a little read through Dwr Cymru's water quality report for 2025 for the Bangor/Caernarfon water supply. It's quite interesting.
I wanted to share the data in a meaningful way, so I pull all the data and put it into an easy to understand infrographic.
Enjoy the read:
Fishkeepers in the Bangor and Caernarfon area are fortunate to have access to one of the softest public water supplies in the UK. Water quality data for the local supply zone show an unusually low mineral content, with a conductivity of approximately 77 µS/cm, a TDS of around 15 ppm, and both general hardness (GH) and carbonate hardness (KH) measuring below 1 dGH and 1 dKH respectively.
For aquarium hobbyists, this means the local water is naturally suited to many soft-water fish species. Bettas, rasboras, tetras, Apistogramma and other fish originating from soft, acidic environments can often be kept without the need for reverse osmosis (RO) systems that aquarists in harder water areas frequently rely upon.
One of the most interesting aspects of the local water chemistry is the presence of elevated phosphate. Water company data show orthophosphate concentrations between 0.87 and 0.95 mg/L as phosphorus, equivalent to approximately 2.7 to 2.9 ppm phosphate (PO₄). This phosphate is deliberately added by the water supplier as part of its corrosion control programme to reduce the leaching of metals from pipes.
Many aquarists associate elevated phosphate with algae problems. However, phosphate alone is rarely the direct cause of nuisance algae. In planted aquaria, algae growth is typically linked to imbalances involving light intensity, photoperiod, carbon dioxide availability, plant growth rates and nutrient balance. Numerous successful planted aquariums operate with phosphate levels between 1 and 3 ppm without experiencing significant algae growth.
The local water profile also highlights an important point regarding buffering and pH stability. Standard KH tests measure carbonate and bicarbonate alkalinity, but they do not account for other buffering systems. Despite KH values below 1 dKH, the supply maintains a relatively stable pH. While further testing would be required to confirm the mechanism, the elevated phosphate concentration may contribute to buffering within this extremely low-mineral water supply.
Aquarists may also notice differences between the pH reported by the water company and measurements taken directly from the tap. This is not unusual in very soft water. Low mineral content means pH can shift more readily through carbon dioxide exchange with the atmosphere. As a result, household measurements may differ from published supply averages.
While the water is excellent for soft-water fish, some aquariums may benefit from remineralisation. Planted tanks, shrimp aquariums, snail tanks and species that require harder water often need additional calcium and magnesium to achieve optimal health and growth. In these situations, targeted remineralisation products can be used to increase GH while maintaining control over water chemistry.
Overall, the Bangor and Caernarfon supply provides a unique starting point for aquarium keeping. Its combination of extremely low TDS, very low hardness and relatively elevated phosphate is unusual within the UK. Understanding these characteristics can help local fishkeepers make informed decisions about stocking, fertilisation, remineralisation and long-term aquarium management.
