Dear All, please can someone advise me on the following. For years I've been keeping corydoras (small catfish) and plecos in a 3 foot tank (127 litres) and they have been fine. Recently I swopped my very large pleco for 4 small ones and I've had trouble with my tank ever since. I obviously didn't realise just how much cleaning up he was doing because the water in my tank keeps going very cloudy and when I checked the stats the Nitrate level is way high - even doing a partial water change doesn't help much because the water from my tap has a high nitrate level. I've got plenty of plants in the tank and try not to overfeed them but don't know whether a NitrateMinus (you probably know the one) product is safe to use with catfish/plecos or whether I should go for something else. Any advise would be very much appreciated.

Sorry, my tank is cold water, 127 litres, all the stats are fine except because I live in Kent the water is hard and the Nitrate level is very high, I use two Stingray filters to filter the water.

Do your water changes with remineralised ro water

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No, I've got to say that I use it straight from the tap - and it's "seen my right" for many years - probably because catfish are so hardy but do you recommend re-mineralised water and sorry for being so densed but how does water become de-mineralised?

matrix in the filter reduces nitrates.My tap nitrates are 20,the tank is less than that with matrix! It takes a few weeks to kick in though.So long as they get no higher than 40,i wouldnt worry too much

What are the actual readings of Nitrate in your tank and in your water source? Most fish are quite tolerant of some level of nitrate, the usual advice is to try and keep your tank water within 20ppm of your tap water. Large water changes will obviously have a quicker effect on the nitrate, but if your levels are extremely high 200+ it would be better to gradually lower it as a huge change in nitrate can be shocking to the fish.
Personally I wouldn't use the nitrate minus, I don't like adding chems to sort a problem as there is nearly always a downside. I believe these nitate minus products can starve the water of oxygen, so if you do use it make sure the tank is well aerated.
Personally I would stick to lots of regular water changes till the nitrate is under control.
Your small plecos will quickly outgrow that tank and they are unlikely to be the source of your problem as the larger specimen you had before probably had a bigger bioload than these 4 combined.

If you use ro water (water that has been filtered through a reverse osmosis unit) it will have around 99% of everthing including nitrate and the beneficial minerals taken out of it so before it can be used for fish the beneficial minerals have to be replaced.
You should be able to buy ro water for around 50p a gallon or you can buy your own unit for around £80 for a basic one that would suit your purposes. If you buy one check the runing cost though

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The sudden change in ph might affect the fish,you would have to do it gradually,if you chose to go RO

If you use tapwater that is laden with nutrients for your water changes, you will simply add to the problem. If the tapwater is high in nutrients, the only way to control it is to filter the water through an RO unit or a Nitragon III. However, the latter are no longer manufactured and are very thin on the ground.

The problem with RO units is the level of waste water they produce. For every few gallons of water that passes through the RO membrane, only a gallon or so will be produced as 'pure water'; the rest is expelled down the drain as waste. This is something you'll need to consider before buying a unit. You can of course buy RO water from an aquatic store in 10, 15 or 25 litre containers, the purity of the water can't always be guaranteed to be 100%.

All aquariums have some detectable nitrate, it is one of the main products of biological filtration. Normally, concentrations less than 30ppm shouldn't have any detrimental long-term effects for the majority of freshwater fish. Critically high levels of nitrate may induce stunted growth.

I would not suggest using nitrate-reducing resins or chemicals, these are rather useless and there isn't really any concrete scientific data to show that these things actually work.

Another very good way to control nutrient levels inside the aquarium is to plant several fast-growing stem species such as Hygrophila. However, since you have Plecs, these might not last very long depending on what species of Plec you have (some are more partial to meaty foodstuffs than plant matter).

Under ideal conditions, the plants will strip the nutrients from the water column and use them for their own growth. Nitrate in particular is used for several things in plants, including the production of amino acids, proteins, chlorophyll, DNA (Deoxyribonucleic acid) and certain other compounds.

Finally, check the use-by date on the nitrate test kit. Nitrate test-reagents have a relatively short short-life, and thus tend to go out-of-date fairly rapidly. An expired test kit can of course give false results.

Thank you - this is all very helpful indeed. My plecos are yellow bristlenose ones which are supposed to only grow up to 6". All things considered I'll test my water from the tap and try the Hygrophila plants first - even if they do get eaten because I was reluctant to use chemicals before reading replies now I'm sure I don't want to use them. Thanks again.

If you don't mind me asking, who told you that Bristlenose catfish were suitable for coldwater aquaria? All species of Ancistrus are tropical, so they should really be kept in temperatures that are no cooler than 24 degrees C.