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!