Hi andyt_uk
It's good to see you back on TFF, but I'm sorry about the circumstances that have brought you here.
I too think it's best to treat your fish with medication for a bacterial infection. Septicemia is bacterial and its often the end result of a problem that might not have been apparent in its early stages. Nipped looking fins and sudden deaths could also indicate that a bacterial infection is going on.
The overall picture leads me to believe that a situation developed that led to a weakening of your fish's immune systems which made it easy for harmful bacteria to take hold once they were introduced to your tank by the new fish. Had they been stronger, they may have been fine with it.
Various factors can stress fish and lower their immune systems. Among these are inadequate and/or infrequent water changes, low oxygen levels in the water, temperature fluctuations (especially fast warm ups), uneaten food in the tank, aggressive tankmates, high nitrates or nitrites, overcrowding, etc. The fact that your fish had recently been sick with another disease would be another stress to their systems that they might not have completely recovered from. Keeping fish at a higher temperature than normal for an extended period would also deplete the oxygen in your water and weaken the fish.
First, it's important to get your substrate cleaned up and the nitrates reduced. If your tap water is naturally high in nitrates, this is apt to take a number of big water changes over the next few days. Don't worry about the medicine; just replace what you removed. It will also add oxygen to the water, but if you think it's indicated, add an extra airstone during their treatment to be sure.
If you do those things and reduce the temperature to no higher than 25 degrees C., you will have done a lot to make the environment less hospitable for the harmful bacteria to survive and reproduce, which will make it easier for the fish to recover.
Here's a link to a thread about Old Tank Syndrome. It might or might not have anything to with your current problems, but since your tank has been set up for a while now, it's something you should be aware of.
http/www.bestfish.com/oldtank.html
I hope your fish recover soon and look forward to seeing you around the forum again.
