RIP "the Little Guy"

@Matt I've never replaced anything on those filters, I just rinse everything in old tank water when doing the weekly clean. 

Poor fishy.    Have you tested the water yet?  What were the results?  Also, do you dose CO2?  Everything points at gas issues to me (either fluctuations in the nitrogen cycle or CO2 dosing).  The hardier fish will survive longer until an organ fails.  Sadly your fish will keep dying until the root cause is found.

Water tests were a ok, but you might be onto something with the Co2 side of things as my dosing has been more erratic recently as I dose liquid co2 but this has been done more heavily at the weekends and a LOT less regularly in the week.  Basically my workload at work has doubled... I was thinking some kind of internal parasite but couldn't match up the symptoms to anything... This makes sense now. Horrible feeling that I might have led to their demise, feel better knowing that I may have found the answer though...

@Sue if I remember rightly Matt unfortunately lost a ram too.  @Matt Have you been using a CO2 drop checker?  They're not too expensive and take the guesswork out of how much CO2 to add to the tank.

Drop checkers work off the amount of dissolved CO2 in the water so it won't matter how it got in there.   

I'm using the EasyCarbo myself but I purposefully under dose against what it suggests.  1-2 ml per 50 l per day is recommended depending on the amount of plants you have.  So in my 90 l I should go around 1.8 ml following the guide.  However, I have low CO2 requirement plants and believe the Fluval Aquasky lights have (very) low lighting at the bottom of this deep aquarium so extra CO2 is not needed .   On top of that I'm not adding large amounts of ferts right now either.  So I only dose 1 ml a day.

That being said Matts lights may be brighter, and fert additions may be greater, than mine so he may need to add more.  The drop checker will let him know when he hits that sweet spot of 30 ppm.

Everyday is a learning experience.  James has been at it longer than I have so I stand corrected.

EasyCarbo needs to be added daily.  Dropping down to half a dose daily sounds good.  If you miss a daily dose don't add extra to compensate on another day.

It's not how it's irregular but the amount of CO2 in the water at the time.  The regularity of dosing is purely for the benefit of the plants.

If you overdose CO2 to a high enough level your fish will get poisoned.  There's not one typical behaviour and from what I've read it's just defined as 'stress' which makes it hard to diagnose.  Some say their fish go to the surface and breathe heavily and others that their fish become lethargic at the bottom.  Your last fish seems to have gone euphoric and then paralyzed.  I've seen a paralyzed glass fish from CO2 poisoning on youtube.  It's front part was clenched and it was going mostly in circles but when he tried to go the other way it looked like an s.  What made me look at gas was the euphoria and then clenched up death that has been described in humans with gas poisoning.

The final click for me was when I remembered reading a forum where Tom Barr mentioned that larger fishes were more likely to suffer from excessive CO2 than smaller ones.  This goes full circle to your first post where your little fish haven't been affected at all.

This is the glass catfish:
<a href="https://www.youtube.com/watch?v=Kp2PFhgTvdo" target="_blank">http://www.youtube.com/watch?v=Kp2PFhgTvdo</a>

To Barr quote:
http://www.plantedtank.net/forums/8-general-planted-tank-discussion/109806-common-signs-co2-toxicity-fish-2.html

I only dosed at the recommended level though... does that change anyone's views?

I confess that the main reason I don't use liquid carbon is because of what it might do to my shrimps......