new tank, dead fish

I bought a new tank, first ever tank and was given very little information on how to keep them going. I was told I could mix all the fish in and that would be that. Great!

But not for long!

My tank is 480x250x385mm (not sure how many gallons that is)

I bought 4 Disus and 6 Clown loaches. As well as 4 black fish, 3" long with a feather like fin running most of the way down the belly. they can swim forward and back just as quickly. (I should tell you I live in China so I only got the names looking through the web.)

Anyway, I let the tank sit for 3 days first with filter running and put some bottled stuff I was given in. These are the steps I have followed which I now know are mostly wrong.
After 3rd day as below.
day 1) Put all fish in at the same time. (wrong i know)
day 2) Fed a little fried food (but discus not eating)
day 3) Fed more, discus not eating, fed frozen blood worms. Discus eating a little but too many so scooped food out. (I way over fed i know now) Also changed 20% water but only from the top and put in water not room temp, but from the tap. (wrong i know)
day 4) 2 Discus fish dying on the bottom, turning black around the fins and specs of black on the body. They are face down and seem to be breathing in the gravel. Removed poorly fish to bucket with new water from tap.
Morning 5) 2 dead discus fish, other 2 in tank not doing so well.
day 6) 2 discus fish eating gravel again, clown loaches laying on bottom of tank all day hardly breathing. Somtimes they are energetic. Mostly hiding behind the coral.

Now, I do not have any other equipment such as thermometer, nitrate or ammonia detector.

What can I do now to prevent a full scale massacre!!! Please can someone tell me step by step what to do!

Please help!