Things you need to take into consideration when buying clown loaches;
a. Clown loaches are very prone to whitespot, they have little immunity against the parasite and many new clown loaches that arrive in lfs/local fish stores will get whitespot sooner or later.
For this reason, you should always quarentine new clown loaches because whitespot is a very infectious parasite and takes a while to treat, you really don't want it in your community tank, so quarentining the clown loaches is very adviseable.
Fish can carry whitespot for up to 2 weeks before showing the classic external white spot symptoms, so even if your loaches appear to be externally physically free of any nasty diseases or parasites, you should still quarentine your new loaches for at least 2 weeks to be on the safe side.
If your loaches do happen to get whitespot during the quarentine period, do NOT use salt, it is very harmful to fish like clown loaches as it causes them a lot of osmotic stress, and since they are not the hardiest of fish to begin with, treating new clown loaches for whitespot with salt can easily finish them off.
Here is the loaches.com advised methods of treating clown loaches for whitespot;
http://forums.loaches.com/viewtopic.php?t=2599
When you treat any fish for whitespot, you should treat it for at least 2 weeks (or depending on what the info on the whitespot med bottle says) and not buy any new fish for a further two weeks after that, since whitespot if not completely eradicated from the aquarium can come back, and new fish which are stressed are particularly vunerable to this parasite.
Your quarentine tank should be cycled before you put any new fish in it, as it would not be good to introduce new fish to an un-cycled tank, so you need to go about establishing some beneficial bacteria in the tanks filter before you introduce any fish to the quarentine tank.
Running the quarentine tank filter in one of your main tanks for a week will cycle it very quickly if your main tanks are already established, after that all you need to do is move the quarentine filter over to the quarentine tank at the same time you introduce the clown loaches.
The bigger the quarentine tank the better, the minimum size you go for depends on what size fish you want to quarentine and how many. If all you are quarentining is no more than 5 small clown loaches (i.e. 2inches or less), then a 5gallon will surfice, how if you want to quarentine more or larger fish, then it would be better to go for something larger.
Quarentine tanks can be overstocked a bit since the fish are only temporary, however that doesn't mean you should go crazy- fish can get stressed from overstocked in quarentine tanks as easily as any other tank, you should also have some strong/good filtration in the tank as well.
b. The second main factor to take into consideration is acclimatisation. You should try to make the journey from the lfs to your tank as quick as posible, and avoid placing the fish in their bag on their journey anywhere in your vehicle that is very hot, cold or suffers a lot of vibrations etc. Wrapping their transport bag in some newspaper or a towel will help prevent their bag cooling down rapidly during the journey, while putting their bag in a cardboard box will help prevent their bag moving around lots on fishes journey.
If you happen to be buying many fish from your lfs, ask the staff not to put too many fish in each bag.
The standard form of acclimatising the fish to a tank is to leave them floating in their bag in the tank water for around 30mins or more, slowly letting in small quantities of tank water into their bag every 5mins or so. If the fish have beeen on a long journey though (2hrs or more) or the ph or temp of the petshop tanks is different to yours, you should quarentine the fish in the tank for up to even an hour.
When the fish are finished acclimatising, turn off the tank lights for the day to help reduce the stress levels of the new fish. If you already have fish in the tank, feeding them while the new fish are settling in can be a good way to distract your current fish from the new ones and allow the new ones to settle into the tank in more peace.
Many clown loaches are not very well fed at petshops, so over the following couple of weeks you should concentrate on feeding them a very nutritious and varied diet (don't over-feed them though, and don't feed too many dried foods as these can be difficult for fish to digest and end up giving the fish constipation)- frozen or live foods are the best, particularly ones like daphinia, krill, bloodworms, artemecia etc. However feeding the loaches on their first day in the tank is not that nesarsary, its best to just let them settle in on the first day before starting them on a new diet etc.
(I will continue in just a moment)