Hello. I have a 40l cold water tank which currently only houses a plec and a weather loach. I would like to introduce something else maybe a goldfish but I don't want to overcrowd or put any fish in danger can somebody give me a little advice please on types and numbers if any at all. Thank you
Really there are no coldwater fish suitable for a 40-litre tank. Your best bet is to return the fish you have and add a heater, then keep a single male Betta splendens in it. If you didn't do a fishless cycle, do please do one after returning the fish and before restocking. This article explains the cycle and how to cycle a tank to make it safe for fish: https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm
Don't feel too badly, I expect like many you've just been the victim of poor advice from a shop. The important part is being willing to do something about it
If you want to keep the weather loach you're going to need a 4ft tank, with sand rather than gravel on the bottom. He will need a couple more weather loaches as companions (they are social fish) and then you could add a good shoal of white cloud minnows or zebra danios. If your house stays above 18c in winter you won't even need a heater for these.