Newbie Need Help On Green Water, newbie need help on green water Options

[gold2dee]

Hello, i have tried out green water using a 150W MH cool daylight bulb for the past 3 mths. I have only 3 goldfish of average size 7cm (body length). They are in a large blue guppy tub. The total amt of water the tub holds is ~170L

I am amazed by the growth and color improvement for the past 2 mths. However, for the past 1 mth, the goldfish seems to deterioriate in health. I see blood streaks in their tail and tail ends seems to tear.

I manage to cure them and re-started the green water but the syptoms always come back within 2 weeks after the water turns green.

If i seed back 10-15% of green water, i realise that on the 2nd day, ammonia is present and will only disappear by the third or fourth day. So i seed back 20%-25% instead and my green water turns dark green in just 2 days time. So every 2 days have to change 80% of water. Shiong ah.

Am i overloading the tub? Or anything i have done wrong? The MH light is switched on for 10 hours and sodium bicarbonate is added to up the KH to about 4dH. And i have cut down feeding those GFs to twice a day only.

Hope some kind soul can give me some pointers. Thank you very much and sorry for the long description.