If it is diatoms, it feeds off of silicates in the water. Are you using RO
or are you using treated tap water? There will always be some silicate when you start up a tank but the diatoms should consume it then die off. If you are using tap you may be adding silicates back in when you do PWC
If it is red algae, there can be many issues causing it. Phosphates are one but again, that would be taken care of by using RO
. Overfeeding or keeping you lights on too long can also play a big part. Water flow is another big part of it. If it is red algae increasing water flow should help.
Hope all that helps.