I really could do with some help trying to figure out what's going on in my aquariums, this will probably be quite long so thanks in advance.

My 80l aquarium consisted of a black phantom tetra, celestial pearl danio, a couple cherry shrimp and a handful of snails up until a couple of months ago. It's a well established tank a couple of years old and the filter media older still, these fish are the last survivors of their shoals, a little younger than the others were.

30/12/18 Added one pair honey gourami and 6 orange shrimp.
6/1/19 Added one pair Agassizi Cichlid - male chases the female also the gouramis, platy and tetra, mostly around meal times.
7/1/19 Added one Platy
12/1/19 Added 6x Daisies Rice Fish and 6x Otto
15/2/19 One ricefish dead, another with cloudy eye - later dies (Ricefish were very active and eating well the past couple days, I assume an acclimatization issue)
19/1/19 Added 6x dwarf rainbowfish
26/1/19 Platy dies - nothing visibly wrong. Had been a little shy over the past few days I thought because of the cichlid
2/2/19 Male cichlid moved to 40l alone, new setup, water and filter media from main tank
6/2/19 rainbowfish gasping at surface - dissapears after a few days
7/2/19 female cichlid dies - pale but nothing visibly wrong, hadn't been well for a while, I thought this was due to the male but she never improved
9/2/19 gouramis make bubble nest
9/2/19 Ricefish dead suddenly - nothing visibly wrong
9/2/19 Otto has fury nose and gill area appears damaged - dies a day or so later
16/2/19 gouramis lay eggs - and eat them
18/2/19 rainbowfish gasping at surface - I realised this time he couldn't sink. After a couple days sunk to bottom and couldn't swim properly then disappeared.
23/2/19 Ricefish laying on bottom - had been totally fine the day before - nothing visibly wrong - not seen since
25/2/19 Male cichlid suddenly lethargic, breathing rapidly, laying on the floor/ leaves/ up against filter intake. Wont eat. Did a very large water change just in case, nothing visibly wrong only pale
26/2/19 No change is male cichlid condition, just getting paler.

Throughout all of this my water has been perfect, 0 ammonia, 0 nitrites in both tanks. 0 nitrates in the 80l and the 40l sits at about 5ppm. pH stable at 7.4. All tested using the API freshwater master test kit, I even bought a new one in case they could go out of date and tested with two other brands of of test kit I had laying around just in case. I treated first for bacterial infection, I've been treating lately with disease clear which is for bacteria and fungus.

I have no idea what to do to stop my fish dying. The rice fish seem to randomly pop off without warning. All fish seem fine at the moment in the 80l, I have no idea where to start with the cichlid.

Any help or advise would be greatly appreciated, I've lost enough fish already.