I lost my pleco last week :( So I thought I would try out a new bottom feeder and went for some green corys.

On Monday, I bought 3 to see how they would do before I added more. Unfortunately, one died the next morning. So I blamed it on the shock on switching over to a new tank.

I took him back to the LFS and got 2 more corys, thinking a bigger group would help them relax more. So of the 4 I had Tuesday morning, I lost 2 more over the night...

I'm lost and really concerned for my fish :( I added 5 small tiger barbs last week and they have had no issues *knock on wood*

I have a 65 gallon tank
Topfin 60 filter (I know it's not strong enough, I'm working on getting a better one)
A few live plants
5 sunburst platties
5 red wag platties
5 black skirt tetras
3 Dalmation mollies
1 Angelfish
5 tiger barbs
2 of 4 green corys :-(

I do weekly water changes of at least 25% and I tested my water this morning after changing it because I lost the 2 corys over night and here's a picture of my testing strip.

http://postimage.org/image/7ytxmg71h/

My nitrates seem high but that's probably due to the dead fish, even after the small water change so I know I need to work on lowering that down.

Any suggestions on how I can improve my tank for my corys? I'm open to any suggestions and would appreciate any feedback. Thanks!