Cory Dying

[Modders]

Hi All, I bought a 240l aquarium in Feb last year and all has been going well up to last week. I'm concerned that I've lost 2 false julii cory in the space of a week, so I was wondering if anyone could help me pin down the problem?
 
The first one was floating upside down at the surface with rapid gill movement on Friday and was dead Saturday morning. This was a cory I bought in March last year. The second I found last night floating on it's side, also with rapid gill movement, it died an hour or so later and I think it was one I bought in November.
 
After I found the first cory I tested the water... Ammonia 0, Nitrites 0, Nitrates 20ppm and I did a large water change. The tests (API liquid kit) haven't indicated any ammonia or nitrites since I set-up the tank.
 
Changes to the aquarium...
 
In November the platys were breeding out of control and always hid in the plants at the back so I decided to take them to the petshop and get more cory, neon tetras and harlequin rasbora. Before I got the new fish, I changed the aquarium from 60% sand / 40% gravel to 100% sand and bought a load of 'dragon rock' to replace some bog wood. I then got the new stock and everything seemed fine.
 
In December I changed my declor from API Stress Coat + to Seachem Prime and put some Purigen in one of the filters. The Purigen hasn't been 'regenerated'.
 
At Christmas my 3 year old nephew bought me a treasure chest air thing. He was really excited so I had to put it in.
 
The Cory are fed Hikari sinking wafers and frozen bloodworm now and again. The rest are fed Nutrafin Max Tropical flake and frozen bloodworm, although the neons also eat the corys sinking wafers.
 
I've tried to give as much info as possible without writing an essay, so if you need more info please let me know.

 

[Modders]

Oh, a couple more points. The first cory didn't have any barbels when I bought him and never grew or acted like a typical cory. The second one its barbel's were fine and acted normally up to finding it at the surface. 
 
I'm not sure if it's relevant but the past few weeks I've been having a problem with algae. I've always had the green algae that covers the glass and comes off easily, but the past month I've been getting small spots of black algae (hair algae?) and small spots of bright green on the bottom half of the glass at the front, both of which are really difficult to remove. 

 

[techen]

How often do you feed them bloodworms?
 
It sounds like swim bladder, Sometimes due to over feeding/constipation or sometimes parasites. Normally feeding corys peas might get them to poop away but I find that since corys are a bottom fish, Feeding peas normally does not work.
 
There are treatments you can buy for it, Worth a shot if it occurs again.

 

[Lillefishy]

Corys probably won't eat peas, mine don't.
Try Daphnia, live or frozen they can help with constipation in corys

 

[Modders]

hey guys, thanks for your help, that's a huge relief. They only get bloodworm once a week, but with the neons eating the sinking pellets I have been dropping in some extra to make sure the corys get enough. I'll cut back and call for some daphnia tomorrow. Would they be likely to eat any other green veg if I pin it to the bottom? It's not something I've tried before.

 

[Lillefishy]

Corys tend to ignore veg, although they are omnivorous they verge on the side of carnivores & need a high percentage of protein in their diet, try getting other frozen or live foods such as brine shrimp, cyclops, bloodworm etc as well as a good quality catfish pellet or wafer.