Hi- I've had a community tank (200L) for about 3 years- all supposedly 'easy' fish and plants but the pattern is always the same- one by one we lose the fish and the plants gradually die. I've asked every fish expert I know, and had the tank water tested (it's fine). It's a consistent 7.6pH-a bit on the high side but this is due to our water treatment system and can't be changed (other than adding a pH adjuster every time I do a water change). The fish always look healthy and active but disappear overnight (presumably they die and are eaten by the others). Over the time we've had the tank, we've lost several dwarf gouramis, rams, tetras, danios and corydoras. The plants gradually become brown and break up. I have the aquarium lights on for about 7 hours a day, and do the usual cleaning/water changes regularly. I'm pretty sure the problem isn't over-feeding. Anyone have any ideas about what could be going on? It's really disheartening. Any suggestions welcome!
Firstly, the tank water test results for water quality are only valid on the day and time they’re done. Ammonia and nitrite need to be consistently at 0, with nitrates consistently at no more than 20 above tap water level. If you buy your own liquid-based test kit for ammonia, nitrite and nitrates, then you’ll be able to monitor this daily to see whether the levels change over the course of the week (assuming you do a water change weekly). If they do, then you may need to increase the frequency and quantity of water changes eg to twice per week at 40% rather than once per week at 20%. It might be that there has been a small, low level of ammonia or nitrite or moderately high nitrates which might be describable as “fine” but would be toxic especially with sustained exposure by the fish over time which in turn affects fish and plant health.
Secondly, of far more importance to keeping fish is that your water parameters (water hardness levels) suit the fish. You can establish this via GH and KH tests – liquid-based ones are far more accurate than paper-based test ones for these parameters. Alternatively, your water supplier’s website ought to give you various results if you input your postcode; the classification of “moderate hardness” isn’t particularly helpful, but take a look to see what the results are for degrees French/German/Clark or mg/l CaCO3 (or Calcium or Magnesium). Your fish are mainly soft water fish so, if your water is not within their required range (seriouslyfish.com gives you each species’ required range), then this will affect their health and lifespans.
What exactly is your water change regime? Sometimes people change all the filter media – this is where the beneficial bacteria live to process fish waste; if you change it or run it under the tap to rinse it (as opposed to gently squeezing it in being-discarded tank water), then this would result in a mini-cycle occurring which again might account for the problems.
How many fish do you have in the tank when it's fully stocked ie how many of each species? This might be helpful to establish if you might be over-stocked which in turn will affect water quality as well as other issues such as insufficient shoal numbers which might stress the fish.
Hope this helps as a "starter for ten" for us to get to the root of your problems.