I have a 30l Biorb, i recently restarted after a heater failure so fish died. I left it 2 weeks to establish and then added 3 platties. They are doing fine, I have now added 4 Guppies. 3 are fine but 1 died after 24 hours. The shop replaced him, however his replacement is definatly dying 24 hours after entering tank.

I have checked water and levels are fine. Temp is between 25-27 degrees depending on time of day. Am i just being unlucky or is there something else wrong? Is 7 fish too many for the tank?

Too many fish added too quickly perhaps? Those tanks are quite small. Perhaps test the water for nitrate levels?

Too many fish, yes. 30l is very small, really only suitable for a betta or a few endler guppies. Test your water to ensure your tank has cycled but check out Practical Fishkeeping, Think Fish and Seriously Fish sites and forums for advice on fish/litre stocking levels.

as the others say too many fish.

Sorry to hear about the loss by the way