Not been on for a while....my fish were doing fine. I wasnt changing water but it didnt seem to be affecting them. I have 2 large angels, 2 neons, 3 goldie looking things, 2 guppies and 2 algae eaters.

I added 6 tiger barbs, 3 tetras, 3 harlequins and 3 days later ive lost 8 fish. my orginal guppies 1 golden one and a neon are dead along with all 3 (red head and zebra tail) along with 2 of my new tetras are dead.

I added new ater 40% but didnt have any treatment left.
is that what will have caused the deaths or was it maybe the new fish being added?? A full EPI water check shoed nothing wrong with the water.

it sounds very much likle you've added too many fish at once and the filter's had a mini cycle because of it

can you let us know what your readings for pH, ammonia, nitrite and nitrate are?

Yes, sounds like what MW says. Its always good to intensify the testing prior to stocking additions to verify that stats are as you think they are and then its probably a good guideline to only add maybe 3 fish at a time, giving the filter some days to grow the necessary bigger bacteria populations for those 3 fish before adding the next 3 or so. During all that you'll want to test closely to see if the filter mini-cycles and do immediate water changes if that appears to be happening.

~~waterdrop~~

Please don't neglect water changes; they're as important for fish you're planning to add as they are for the fish already in the tank.

How long did you leave between water changes (honestly)?

If it was more than a couple of months, say, you may be facing 'Old Tank Syndrome' in which case water changes will make things worse. It may have been your 40% water change which killed your fish.

As Miss Wiggle says, post up your readings for Ammonia, Nitrite, Nitrate and pH and we will be able to assess the situation better.

In case you have never heard of old tank syndrome, the concept is simple. If you don't do water changes for a long time, the water quality deteriorates but the fish that are in the tank get gradually acclimated to it so most of them survive. They have been able to keep up with the change. When you add a new fish, or several, the new fish has not had a chance to get used to the poor water quality so it gets distressed and some of them die. Now the really bad part. When you figure out that by not doing water changes it is hurting the new fish, you try to help by doing a large water change. What happens is it helps a bit with the new fish but the old ones now have to adapt rapidly to good water and many of them fail to do so. Result is more dead fish. The best thing you can do with old tank syndrome is to avoid new fish and do lotsw of small water changes over a couple of weeks. As the water quality starts to improve, the water becomes safer for the fish that are already in it and it becomes safer to add new fish.