Plants going brown & algae

Hi

I have had a tank setup for about 10 weeks and up to recently the plants have been flourishing, even cabomba has grown which is something I could not get to grow before. The crypts and pygmy chain have put out loads of runners. However, over the last couple of weeks these plants have suddenly started going brown and dying and the cabomba is going brown at the top. I also have algae on the plants and glass but not excessive. What has changed is that over the last three weeks I have added about 20 fish, nothing exessive just tetras, coolie loach, dwarf gourami and dora catfish. Nitrite is 0.1 and Nitrate 20 mg/l and Ammonia 0.6, all of which have been constant for 6 weeks. GH is >16, KH is difficult to read but I think it may be about 10, PH is 8.0 but none of these have changed since the tank was setup.

I have 2 x T8 tubes (standard manufacturer mix) and keep the lights on for 12 hours per day. I work in the same room (10' x 8') with 3x50w spot lights but minimal impact on the tank. The room is naturally dim.

Has anyone any idea on what may have changed. I never used to be able to keep plants so I was really chuffed when everything started growing. The tank is 33" x 13" x 24" and 200l. Substrate is sand and I have a huge piece of bogwood in it.

Thanks
Steve

 

NO, I didn't see a need to add anything as the plants were growing really well. What I don't understand is that the pygmy chain swords were perfect with runners spreading like anything. They had been in the tank for about 6 weeks then suddenly they are all going brown and the runners stopped as well. What else should I measure before I start to add CO2 or Excel?

Thanks
Steve