Angel Fish Swimming On Their Side/upside Down

Ok so I have my 180lt jewel rio set up with 7 hand reared angles, some plattys, 2 small loaches and 6 tetras in, went in to feed them this morning and one of the angles turned upside down in front of my eyes, then notices another one stuck upside down, they have swollen bellies and are appearing to gasp for air.
Did an intimidate 70% water change, and added some fresh bacteria before heading to work.
I have come home and the two are still the same hanging upside down or on their sides gasping, I have checked the water and all the readings are negative for toxins.
I have added some general tonic to see if that might help, but beyond that I am stumped, I have never seen anything like this before, and am very concerned.
Also non of my other angles want to be anywhere near the 2 that are affected?!?!?!
Anyone any ideas please, I really could use some help???!!!!

I've been doing a lot of research trying to find an answer, and the thing it looks most likely to be is bloating from constipation, as they are not sinking and struggling to get to the top they are just floating on their sides with swollen bellies, I have isolated them and will try starving them then feeding them blanched peas, all the others are fine.

Their scale are flat, and other than floating on their side and being bloated, they look in good condition.

Well we have just checked the tank, and one of the angles has ermmmmm.....well, passed what ever it had to, which is good news!! The other one is still hanging at the top of the water but is at least no longer on it's side, it's now upright.
I can honestly say I have never had constipated fish before!!
Thanks for the advice and support, I will keep you updated with how they go on, I will be feeding them blanched peas once a week from now on!!

Apologies for late reply but just to add that, having had extensive experience of swimbladder-type problems in goldfish, I would always advocate dietary changes as a first-line treatment - and, for serving peas, best to take the shell off and cut each pea half into tiny morsels, then hold that under the filter outlet which should cause a shower of pea pieces and increases the likelihood of everyone getting fed.

My final update on the two bloated angels, they are both now fine, whatever was stuck has passed!!! I have fed them for the first time in 3 days on very healthy blanched peas.
The only trace of anything wrong, or signs of what happened are that the angel who took longest to recover now has slightly shortened fins, due to them being out of the water for periods as he floated on his side
However these will grow back, and I will in future be much more mindful of what and how much I feed them so that I can avoid this happening again.
Many thanks as always to you all for you advice and encouragement in treating them