Thanks for your replies. Sadly, the Red Rainbow, ended up having white fluffy muscous looking poop, so we actually euthanised it with clove oil. Our second Pearl died the following morning. The Red Rainbow, was probably quite old, so could have been old age for him. We are thinking maybe a parasite or infection of some sort though. We have not seen any evidence of fish attacking or being nasty to each other.
We still have 1 x Siamese Algae eater, 2 x Rainbow Bozemans, 2 x Giant Danios, 2 x Venezuela cory's, 1 x red Whiptail, 6 x Cardinals, 1 x Starlight Pleco, 5 x zebra loaches.
Our water is as follows:
Ph 6.8
Ammonia 0
Nitrite 0
Nitrate 40
We have just bought some aquarium salt to put the fish into for a brief time and also will get some methylene blue to treat the tank. Tonight we cleaned the filters as our water was starting to have very small bits floating around rather than being crystal clear like it used to be.
Thank you again for your thoughts maxxi1 and Caryl.