Ok well here's the deal... I have a betta in a small 2L tank (just temporary) and next to him I have a big 40 litre tank which I am starting up as my tropical tank... I just started by getting 3 black widow tetras and in 1 day 2 have died! (1 seems absolutely fine). Every fish I had in that tank has died including a betta :( the water has been tested (fine) and the betta is in the same water frOm the same tap treated with the same conditioner and I have had no problems with him whatsoever!!! The tank has been rinsed (left running for a while then emptied) multiple times, it has had salt water as well as fresh water rinses and has been scrubbed!
What is going on with the big tank????
(40L JEBO tank with filter, heater, gravel, 2 plastic plants, 1 plastic rock cave thing, and [had] 3 angel fish, then 1 betta, then 3 black widow tettras [only one of which is still alive])
Please help before my last tetra dies on me!!!!!!!!!!!!!