Tank sizes are for adult fish rather than groups - the example you gave means Zebra Danios as a species can be kept in a 10 gallon minimum and they should be kept in groups of at least 6. However... Zebra Danios are often cited as a beginner fish which is why the small tank sizes are often recommended. And based purely on their adult size a 10 gallon should be ok but when you factor in activity levels, its not, they will survive in there but will only thrive in a longer tank because of how active they are. Zebra and Leopard Danios are actually the same species Danio Rerio so they will school together just bred for different markings.
One thing we have not really addressed is that you are over stocked right now too, have you heard of inch per gallon? Its a very rough guide to work out your stocking levels, it is flawed and there is wiggle room depending on species but its a useful starting point. At the moment you have 7 2 inch fish and 1 5 inch fish (though I know you know about the CAE) which puts you at 19 inches in a 6 gallon tank - or without the CAE you are at 14 inches. With the nano species I mentioned that get a little under an inch a school of 6 would put you at fully stocked and you'd be keeping a fish that will do ok in a smaller tank. A few other options would be a trio of Badis - Black Tiger Badis are very cool, or a few male Endlers or a couple of species of Rabora might be ok like Emerald or Galaxy.
Unheated tanks in the UK are generally not a great idea as over winter we get into negative figures. Just think how cold it gets on a night and most people don't have heating on over night as you are tucked up in bed, especially this year with the prices. Would a small heater not be an option? You can get small ones for £10?
Wills
One thing we have not really addressed is that you are over stocked right now too, have you heard of inch per gallon? Its a very rough guide to work out your stocking levels, it is flawed and there is wiggle room depending on species but its a useful starting point. At the moment you have 7 2 inch fish and 1 5 inch fish (though I know you know about the CAE) which puts you at 19 inches in a 6 gallon tank - or without the CAE you are at 14 inches. With the nano species I mentioned that get a little under an inch a school of 6 would put you at fully stocked and you'd be keeping a fish that will do ok in a smaller tank. A few other options would be a trio of Badis - Black Tiger Badis are very cool, or a few male Endlers or a couple of species of Rabora might be ok like Emerald or Galaxy.
Unheated tanks in the UK are generally not a great idea as over winter we get into negative figures. Just think how cold it gets on a night and most people don't have heating on over night as you are tucked up in bed, especially this year with the prices. Would a small heater not be an option? You can get small ones for £10?
Wills
