Hi! My son, Cameron (11) has just got his first fish - 2 glowlight tetras, 2 neon tetras and 2 x-ray tetras in a 25l tank, with marina i25 filter, roof light and heater set at 25 degrees. We have a couple of live plants, a couple of ornaments and gravel in there too, all rinsed prior to use as per instructions. Tank was set up days prior to fish arrival, bag left in tank, then tank water mixed in, etc. Also added tap water treatment. Fed with tropical fish flakes. Got them Saturday. This morning one neon dead, tail first in filter inlet, one x-ray dead (upside down)at bottom of tank, furthest point away from filter, no visible reason. Also one glow light at furthest point from filter with damaged fins and struggling to breathe. The other glow light has damage to top fin but otherwise ok. Have switched off filter for time being and most poorly glow light now swimming to top for air but generally happier. Worried about leaving filter off - should it be on all the time/how long can it be left off for and how frequently? Worried about further casualties if we switch it on. Any advice GREATLY appreciated!
They most likely died and were pulled towards the filter afterwards. The filter needs to stay on all the time to support the healthy bacteria that should be filtering your fishes waste. Unfortunately your filter won't have had time to grow the bacteria you need and is not doing its job, hence the dead fish. The tiny tank isn't helping either, its very difficult to keep such a small body of water stable and healthy even with a mature filter. Ideally a beginner tank would be much larger to help mitigate these problems.
The best thing you can do now is get a large food safe box (or a bigger tank, think 60 liters+) and move over the fish and existing water then follow this guide:
They are sick from toxins in the water. The filter has not been cycled so the water is polluted with ammonia and nitrite from the fishes' waste. These toxins make them unwell (gasping for air) and cause them to suffer problems such as fin-rot amongst other things. The tank is too small to cope with these fish (25 litres is too small for any fish) so the pollution will be worse than it would be in an adequate tank.
To be truthful the best thing you could do for these fish would be to return them to the shop, but as they are sick that may not be an option. The alternative is to get them into a larger volume of water as fast as possible (a plastic food-safe box as Onyxia suggested is a cheap option) and follow the guide to dealing with ammonia and nitrite spikes linked to in Onyxia's previous post.
You will need to buy a good liquid based test kit to monitor the water quality and ensure you do enough water changes to dilute the toxins while the filter cycles. This may mean changing 25% every day (sometimes more) for around 4-6 weeks.