Dont Know What I Am Missing???Please Help

Question
I Got A Tank Size(ft)front.I Got It Three Days Back And I Have Installed A Heater and Water Filter.My Pet Store Owner Advised Me To mix Two Medicine When Changing Water(Drops) ...I DID...Then He Advised Me To Put...Koi Fish(two),Moonlight Gourami(two),BlueLine Shark(Two).All Small.But after 1 Day One Of Koi Fish Died And Other Stopped Moving And After Two Days The Remaining Koi Also Died.One Of My Friend Advised Me To Clean My Tank Completely And Then Put Fishes In It.I Did That But Haven't Got Any Good Result.Now I am a Beginner And I Am Totally Confused What Should I Do.. Temperature in Tank Is 28c. Almost 20Gallons Water. Please Guide Me What Steps Should I Take... 1)Which Fish Should I Keep.. 2)How Much Should I feed Them And When... 3)How Many Fishes Should I Keep And For How Long I Should Not Add Anymore Fishes... Right now I Have Total 6 Fishes..  

Answer
Hi!
First, you must let your tank cycle with no fish in it.  To do this you buy a used sponge that fits your filter from the pet store.  A used sponge is full of good bacteria which fish need to survive.  Squeeze out the access water, and put it directly into your tank.  Do not rinse it under tap water.  (Never rinse a sponge under tap water when you clean your filter.  Always rinse it in water that you have removed from your tank, once your tank is established.)  Ask your pet store to give you a handful of gravel from one of their used tanks.  This too would have good bacteria, and would help the cycling of your tank.   Let your water run for one month or so, only adding water that has evaporated.  Buy an ammonia, nitrite, and nitrate kit, and a PH kit.  It is very important that your ammonia is zero, nitrites are zero, and nitrates should be low.  When you have reached this point, check your PH.  Not all fish live in the same PH, so I cannot tell you what fish to buy at the moment.  Do not add fish until your water chemistry is right.  Do not add chemicals of any kind.  They do not help, and often make things worse.
Goldfish are cool water fish, and do not belong with Gouramis.  One goldfish may grow very big, therefore needs a big tank.  A BlueLine Shark also needs a big tank of 75 gallons or more.  The gouramis are tropical fish, and need heated water.  These fish do not belong in a 20 gallon tank, and they do not mix.  Vendors don't care what you put in your tank, they are there to sell, and make a buck, so always read up on the fish you put in your tank.
Once your water chemistry is right, you may add one or two fish, wait one week, do a 25% water change, check your water again, and if  you have zero ammonia, zero nitrites, and nitrates are low, you may add two more fish, depending on the size of the fish you buy.  Never overstock your tank, and once you have the right fish, you must do water changes every week of 25% to make sure your water chemistry is always right.
Write back to me once your tank has the right chemistry with the PH results, and I will be able to tell you what fish to add, what they eat, what to put into your water to keep it safe.  For now, you must let it cycle.
Hope this helps.
Lynda