hello
responding to the first post will read the thread after
I would recommend increasing the amount of water you change to at least 30%-50%/ week. Personally I do 50% and 2-3x50% on my tanks over 200g.
For water conditioner I find that the biggest bang for the buck is seachem prime since the volume treated per bottle to the cost of bottle ratio surpases any other product in my mind.
I am also a big fan of moving air in my tanks, while there are those who say having a bubbler does nothing, I find a good airpump and diffuser does increase flow in the tank and have done tanks without and with just to compare with the only difference being the addition of the air and I find that my fish are much more active with air in the tank.
I lose fish to agression not so often but it happens, I just lost a loach that swam into the intake tube of my filter and promptly died. I noticed it when the fish started to breath hard, did an inventory and sure enough he was dead in the tube (aside prime will enable you to nuke any inflated water chemistry levels without harming the fish in such a case as this).
And of course I lose fish to getting eaten by other fish since fish eat fish.
There is also the rehoming factor when I take fish from other peoples set ups where the water condition is shall we say not the best. For example here is a picture of the set up that I just took the remaining fish out of the other day
this is one of the fish, a jack dempsy without normal coloring
In all cases I find that a regular large volume waterchange keeps the fish happy and alive for longer.
edit:another suggestion would be a regular monthly cleaning of the filter housing and tubes, with the sponge being squeezed out in a bucket of tank water to clear it of excess waste and the filter being cleaned with a brush for a dirty filter will defeat a waterchange any day