“I started off with 2 neon tetras and 3 corydoras, they’ve lived for about a year fine, i bought 5 more tetras and about a week later i found one dead, i then went and bought 6 new guppies and I’ve noticed they stay at the top it looks like they’re gasping for air, anyway it’s been two days since i out them in the tank and this morning i’ve woken up and found my original tetra, original corydora and one of the new tetras dead.
why has this happened’
is it an oxygen problem’ do i need live plants or an air pump’
at the moment I have a marina i25 filter which i dont know whether its powerful enough

3 hours ago
– 4 days left to answer.

Additional Details
by the way my tank is 35 litres

1 hour ago
and I also have a heater, the tank cant be too small as the tanks in the petshop are the same size

1 hour ago”

and everyone said theres a fish overload, so my new question is now i only have 12 will they survive or do I HAVE to get a bigger tank’
also I bought some plastic plants off ebay rinsed them in hot water and placed them in, and then they started dying, could it have something to do with this’
Thanks everyone so much!! i think the filter was too low down as it was below water level so i moved it up this morning and when i came back from work they are all very active and seem very happy!! I’ve taken the plants out too.
also just to let you know, the man at the pet shop said these combinations would be fine, otherwise i honestly would not have bought more, and as for the plants i added them the same day i added the first new batch of fish this is why i didn’t know whether it was overload or toxicity