Dying fish

[black angel]

i would say you have a bacterial disease in your tank. Only changing the water every few weeks may not be helping either as your fish grow so the effect is felt on the filter and bad bacteria can get out of hand which arent shown on test kits.
You should do more frequent water changes.

 

[WILDER]

Read this article do a water change and some salt baths on the affected fish, add some salt to the tank if you have no scaless fish.
http/www.flippersandfins.net/flexibacter.htm
http/ukdiscus.co.uk/columnaris.htm#Ident...ng%20columnaris

 

[webcat5]

You said that your water hardness registers as soft?
Most of the fish prefer medium to moderate hardness, once you get that right your algea may disappear as well, mine did.