Unknown poisoning??

Question
Have run a tepid water aquarium for a year (no tropical but temp regulated to 21 C) but suddenly lost all fish (rainbow minnows , sucking loach and orfe)except for 4 inch Plec. Left tank for month and increased temp. to 26 celcius.  Tested water - 0 Amonia 0 Nitrite and less then 40ppm Nitrate.  PH 7.6-7.8 GH approx 16 Degree GH.
Introduced 4 guppy and 2nd Golden nugget plec.  Guppy's deied within 36 hours. Seen to be swimming at surface night before. Red gills - intitial thought poisoning. Amonia and nitrite levels still 0. Large scale water change and cleaned objects and waited for another month. Was recomended by shop to have 5 guppy's, 2 sucking loach and 3 sordtails.  Within three hours loachs swimming badly and died and others at surface (one guppy died and outlook not good for the others). Water still tests ok. What could be poisoning them so quickly? could it be clorine? all is treated with tapesafe product. At complete loss as Plac's appear unaffected.  Had new windows fitted prior to loss of tepid water fish and wondered about the solvents in the sealant used, but the water has been changed many times since.  
Any view welcome, Thanks

Answer
Hi Eliot;

What size tank is it?
How long do you usually acclimate the new fish before release into the tank?
How long does it take to get the fish from the store into your tank?
Are these fish all from the same store?
Do you put the transport water into the tank or do you net the fish from it and throw it away?
Has the plecostamus been chasing the new ones?
Have you had your test kits for awhile?
What kind of kits are they? (drops, tablets, dip-strips?)

Let me know as soon as you can...

Chris Robbins