My first casualty - any ideas on potential cause appreciated

My female ram died today - only had her a week. She is the first fish that I have lost, and I am gutted.

I bought the pair of rams just over a week ago. For the first week the male chased the female constantly and she kept hiding. I was worried that he was harrassing her too much - particularly as he wouldnt let her feed.

I watched her very closely for the week, and tried to ensure that I waited until she was close before adding food, to make sure she got some before he chased her off.

Over the weekend she looked a lot more relaxed and didn't swim away from him -a complete change in behaviour. I thought that she was perhaps thinking about breeding. Her colours looked stronger, and we finally thought the pair had settled in.

This morning she wouldnt come out for food. She was hiding in the back corner of the tank behind some bogwood.

This evening, my wife came back from work to find her at the surface of the tank, seemingly gasping for air. She then died very shortly afterwards.

Couldn't see anything obvious wrong with her at all. All of the other fish in the tank, including the male ram, are looking fine.
Just checked the water chemistry and everything still looks within normal ranges.

I know with this little info I wont get to an exact diagnosis, but a list of possible causes would be really helpful so I have some idea what might have gone wrong.

I don't know whether to get another female or not, and would really appreciate some thoughts.

Thanks in advance. :sad

Full tank history and specs are in the signature link.

Temp, and water chemistry (ammonia, nitrite, nitrate and pH) all checked immediately this happened, and all unchanged.

Also, the cardinals, ottocinclus, cories and the male ram are all also supposed to be sensitive to changes in the above yet all seem fine.

Didnt have a chance to check whether the gills were reddish - what would that signify?

In terms of food changes -
I usually feed flake and catfish pellets, but the rams initially wouldnt eat either of those, so I have been feeding a little frozen bloodworm every other day since putting the rams in just over a week ago.

the evening before the Ram died I put 2 small algae wafers in for the first time - all the fish, including the female ram had a good dig at them and pushed them most of the way around the tank.

As hound said, I had heard that Rams pair for life, hence had only bought one male and one female.

Do fish tend to go downhill that fast with intestinal parasites or stress? Also, how catching are the parasites - can I expect problems with the rest of the fish in the tank? They all looked ok this morning.

Thanks for all the comments so far.

Sounds like adding another lone female is probably not a great move then.

Is the male likely to be ok on his own?
At the moment he seems fine, but I dont want him to develop antisocial or unhappy behaviour