Need Help With My Tank And Goldfish !, algae ~ and fishes Options

[javanoob]

hi

i have a 2 feet tank..
4 goldfish, all babies or quite young. ard 4cm to 5cm including tail.
i have 2 air pump, 1 ehimen external filter. 1 overhead light and some rocks and decorations for bacteria growth.
inside the filter, there is bio rings and ceramic .. the standard setup...
i change 65% water every 3 weeks.
i have been rearing the fishes for ard 4mths, everythings fine..

the readings for amonia ,nitrate and nitate and ph, is fine as well.. someitmes moderate..

i just change my fish water ard 1 week ago..
and now i realise..

at the back of the tank, there is brownish patches algae growing.. its getting more each day..
for the 4mths i have nothing.. why is it growing now ?

what should i do to stop the growth , its ugly..

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my goldfish has not been as active as they are.. they are swimming slowly, most of the time sleeping.. when they are awake,, they will just keep digging the gravels, swim ard ..
but they are very very active during feeding time..

is it normal ?