desperate!

Let's start with this stuff;   Tank size: 5'x2'x2' pH: 6.5 ammonia: 0 nitrite: 0 nitrate: between 20 - 40 kH: and gH: not sure, but our water is very, very soft tank temp: 24°C   Filtration is two APS 2000lph, 50 or 60% water changes weekly. Other occupants of the tank are a synodontis, one...