lightleaks surely are a big problem , but the main culprit here is temperature !
cyanobacteria is your problem here. they DON'T NEED LIGHT to grow, and they love oxygen and all sorts of organics.
they can survive safe concentrations of chlorine and H2O2.
The only way to deal with them for once and for all,
is proper cleaning and maintaining water temps below 21°c all the time (ideally 17.5-18°c).
The reason being is those bacteria strains can't bloom under those temperatures
Afterwards, adding compost tea once a week to introduce beneficial bacteria.
Also one tip is to remove airstones since that's from where they enter yr reservoir.
Airpumps not only bring in pathogens, their test results regarding Dissolved Oxygen where dramatic !
You only need a waterpump on the bottom of the reservoir which is aimed to the surface.
the water surface will swell and create more DO than airpumps - all the downsides.
As last, as mentioned by other people, blacken out your reservoir and fix lightleaks !
But without fixing temperature, you will def fail again, that's why it ain't the culprit !
Succ6 beating cyano, i did it so one day you will to
PS: loads of peeps will tell you to buy bigger airpumps and stones and all sorts of crap, don't listen to that !!
i believed all this crap and kept on failing over and over again ! save your money and buy a chiller.
the only workaround here is mentioned above.