On Monday I added 3 otos to my 20g long semi heavily planted with new plants that only had a betta (recently moved out 3 other otos to a different tank when new plants arrived for 20.. original otos are fine). I stupidly decided to not quarantine the new otos, and to put the HOB that was on my 10g for a month back onto the 20g without cleaning it first (10g houses some shrimp until last week.. but found 2 more live and healthy ones in there yesterday... sneaky buggers) and put an almond leaf that was in the 10g into the 20g...
Otos were drip acclimated and added, lights off in tank for 18 hours. I added a large dose of Prime to the tank incase of an ammonia spike form the new fish. Next day I found a dead oto.. wasn't too surpised the moron in the fish department litterally droped one on the floor ad still made me take it (refused to take another out of the tank) and also nearly squished 2 trying to get them into the bag.... well blah blah found another dead oto today (again assuming beuase of miss handling at petstore) both otos were pale when I found them..I'd checked the tank 4 hours ago and the most recent oto died between then and now and lost color. Then I noticed my betta is very lethargic, hanging out at the surface in some plants, and his face is... I don't know how to describe it just look:
(and yes he is still alive in the photo)

his face was a dark grey/green hue before this happened
Betta is staying at the surface, he is not breathing heavy (no fast gill movement) but is taking periodic gulps of air, no clamped fins, not eating (but I think he can't see food now), very lethargic but will move if touched. These symptoms only became visible today, he was swimming and eating normally yesterday evening.
I did not see this sort of white stuff on the otos but I did not see much of them after putting them in the tank.
tank stats:
large hob (rated for 70g) + airstone always on in 20g (smaller hob rated for up to 20g tank added monday)
temp 78F
ph 7.5
gh 160 kh 70
ammonia 0
nitrite 0
nitrate 20-40 range
substrate inert black diamond
Tank has been up and running for a 2 months, with seed media (whole filter's worth) from an 10g tank that was running since 2012, had the (original) 3 otos and betta in from beginning of April.
I should mention when I bought the otos at the fish store, 2 went into another tank (no other changes to that tank) they are still alive and show no signs of disease, nor does the other tank's betta.
The otos that died in the 20g hid mostly (assumed new home so they were scared) when I did see them when alive they still had color though.
I don't know if there is still 1 live oto in there hiding or a corpse stuck in the denser part of the plants.. will having to go 'fishing' through later to see
I'm not quite sure whats causing this but I'm guessing either I just 'lucked' out and got a sick oto (not whole batch) that's spreading through the 20g, or that the HOB and/or almond leaf from the 10g brought something back over to the fish tank that did not effect cherry/rili shrimp but is killing the fish.
So anyone have any ideas wtf did this to the betta (and k.o.ed the otos)?