Some species of newt are protected like the golden crested newt so you couldnt move them anyway. Newts may not even host your pond and end up going back to the neighbour! You just have to hope they will spread to your pond if they are protected.
Welcome to FK! Yes definately take him back. The tank is far too small for even one goldfish let alone 3, they grow extremey large and need tanks starting from 30 galons for one fish and filtration rated for a tank twice the size. Try and take the other two fish back aswell ASAP! I cannot stress this enough. Your LFS will tell you that your current stocking levels are fine as when your fish get ill which they will with that water quality and tank size they can sell you a buch of meds. Then when the fish dies they get you buying more until you get fed up. What is your water change routine? It should be 15-20% water change everyweek using dechlorinated water, then every other week the filter media should be washed in old tank water. I would suggest a 30% water change straight away, or even better take the fish back now! Nitrate should be kept under 40ppm and nitrite should always be zero as the fish will be struggling to absorb enough oxygen into their gills. This also stresses the immume system making them vulnerable to opertunistic infections such as whitespot. Let us know how you get on as I know this will probally be fairly contrary to what you have been told by your LFS but we see this a huge amount on FK and can offer unbiased advice that is best for both you and more importantly your fish!
The main reason why so many people keep them as they are advised by their LFS to get them as snail eaters or they are not told how big they will get, its easily done as you have found out but many dont get to their full size, they should live to 20 years. Peas help with most fish to relieve constipation so it wouldnt hurt either way. I would definately consider taking them back to the store TBH unless you are looking at a tank that size, they are also sholaing fish so need groups of 6+. I will try and find you a link to a really good site on them later.
It really gets on my nerves how often clown loaches are sold to people with inadequete tanks by the LFS. Unless you have a tank of 500+L preferably 700L then take him back to the store as they get very large (30cm) and are shoaling fish so need groups of 6+. If the others are nipping at him then yes it was but make sure he has enough room. As for the danio you often get one that gets picked on hence why shoaling is advised as it spreads any aggression. Most likely it is that he has been picked on and his fins should grow back. Have you got a picture of the clown loach and the danio. Also I know they are only with test strips but what are your EXACT water readings. I know maidenhead say to treat clown loaches with protoon (sp.) after purchase as they can get quite stressed.
The reason why people generalise about fish behaviour on here is that the vast majority of whatever species they are talking about display that behaviour. I got a dwarf puffer who was absolutely fine for 6 months and then all of a sudden he killed one of my danios, I was told it may happen and yet I used your advice of seeing what works. But as you said these are living creatures so it was extemely stupid of me putting in a DP when there was even a small chance of it killing on of my fish. If they are incompatable, dont do it saves alot of hassle.
As a newbie myself i have learnt never to say happy behaviour. It is easy to see stressed behaviour but "happiness" is not easy to tell. I thought my danios were "happy" in there small overstocked tank untill I moved them to their own much larger tank and are now completely different fish, just because the fish arent shoaling together or tracing the glass doesnt mean their not stressed. If in doubt go for the most ethical option, which isnt 12 fish (half of which will get fairly big) in a 60L.
I have a couple of similar but slightly smaller about (1cm) similar looking things but with wings in my puffer tank. I feed him live bloodworm and so is it possible for them to go through their life cycle and become red mosquitos in a tank? If so could they be harmful at all?
You can get play sand really easily all year round. Instead of it costing about a ?1/kg in my LFS it cost just under ?3 for 12.5kg at Argos. The sand looks so cool as all that is in the tank so far is MTS so there are loads of snail trails in the substrate. What type of gravel have you got matt. I think with most other types of substrate you have dont have much choice other than the rip off LFS stuff.