Hello and the issue!

[Barbless]

Ok, I have been reading your forums and info for a while now so thank you to everyone who contributes it does help make sure people are safer in their aquatic behavior (ok me!)

I am a beginning aquarist who has had a small 10 gallon tank for 3 years prior to trying a more adventurous step

I do have an issue/problem which I can't seem to figure out a solution to which basically relates to fish dying. I have had my larger tank for ~8 months.
I had started moving from a 10 gallon tank to my 45 gallon when I decided to expand my fish base. I started with moving the 5 guppies (kids loved them and they can be pretty; one female and 4 males) and a chinese algae eater in addition to adding some plants (which came with snails). I waited a week and then added a pleco and ghost shrimp. Aside from one algae bloom things continued to do well. So in a month I added 6 Danios (3 zebra and 3 leopard). Things were good for a long time. The occassional guppy baby would suddenly appear and last for a week or two and of course I cant seem to keep a ghost shrimp alive longer than 3 months... then the trouble

One day the female guppy was dead, She had seemed fine prior and then gone. I removed her and continued my routine treatment of a 10-15% water change every 1-2 weeks (where I add cycle, nutrafin aquaplus and waste control with a s/a salt). The water tests had 0 ammonium and ph was 7.4. I then lost 3 more guppies over the next few weeks which may have been related to the danios chasing them more I'm not sure. I then went and decided to get more fish and something which would utilize the middle of my tank.

I therefore purchased 9 Barbs (4 Tiger, 3 Albino and 2 Moss) as well as ghost shrimp. Then more trouble began, within a week I had lost the last guppy, 2 Moss and one Tiger barb. I took the Barbs back with a water sample (my check was PH 7.5 and ammonium was between 0 and the next value on the chart which I thought was reasonable). They felt the water was ok also. So I got replacements and placed them in the tank. Within a week one of the Moss died again and this time my Danios began dying. Within the last 2 weeks 5 have died. I have continued my same changes, the water values appear fine and the ghost shrimp are still alive...

I'm just not sure what I should do or check. I have never seen any concerning behavior except for 2 of the barbs did point down a bit prior to being found dead the next day. I see no obvious diseases mentioned in the fish disease section and my water testing cannot find a significant problem. I'm at a loss...
ps sorry for wall of text!