Ive had a read through your historic posts.
You have very high pH, water company reports it as high as 9 pH. You also have essentially zero general hardness(
GH) and carbonate hardness (
KH) which was surmised as being responsible for pH crashes in the tank and you having difficulty cycling the tank.
You now are seeing diatoms.
Diamtoms feed on the silicates and nitrate. They normally show up in newly established tanks and feed on silicates on new substrates and new tank assembly and once these silicates are gone they tend to die off. When they stick around it tends to be because the silicates are being replenished and not doing sufficient water changes to remove nitrate.
Whats your nitrate like?
I have a couple of theories.
- You tap water is high in silicates. Silicates will raise pH, silicates wont contribute to
KH, and some silicates dont contribute to
GH. This kind of makes sense.
- Or the buffer you are adding is the source of whatever you diatoms are feeding on.
You have tried algaecides without success. Ive not heard of any algaecides that have an effect on diatoms.
Things you could try.
- Increase the lighting. This will promote green algae that will outcompete the diatoms. Swapping one issue for another, but green algae is easier to control.
- Get something to eat the diatoms. Nerite snails are known diatom eating inverts (although mine have never eaten diatoms). 4 Otos cleaned a 15g tank of mine that was covered in diatoms in a little over 24 hours, after which they seem happy with vegetables and algae wafers.
- Remove the silicate source. Does the water report you got indicate high silcates in your tap water? There are chemical treatments that will reduce silicates. I usually steer away from chemical treatment, but yours may be an extreme case
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