
most fishstores will tell you things are fine to sell you more fish....id go on the assumption that the tank isnt cycled after 3 weeks unless you used established filter from another tank, or tetra safestart....so they are probably having an ammonia or nitrite issue...im thinking ammonia though because 3 weeks is a short period of time and probably just the start of ammonia rising...if you can get some prime or stress coat+ and do daily water changes until you can get your own test kit, that would help them alot...an
API liquid master test kit has all you will need and is good for alot of tests..goodluck!