Yes.
the only change was a reduction in plants.
The change was a reduction in plants, the effects of that change will be felt across the tanks assemblage very suddenly. There’s also a minimal amount of plants in the setup. Might be related, equally it might not.
The cause is most likely what the others already surmised.
Compared to the above comments, have found BGA to appear at startup when there is a lot of change going on. Have also seen it suddenly appear in a tank that is a year old running constant with inputs, no interruption to filters and minimal trimming back. It has never been as simple as ‘this’ caused ‘that’ from personal experience and if you wipe it out, then it doesn’t return despite changing nothing about the setup. How does this work?
Some believe it gets a foothold due the ratio of N to P. Can’t say this is much use in an aquarium with active substrate, but may be useful information that can be pursued by those using water column dosing with inert substrates.
If the design and running of the setup is key to why it appeared in the first place, why wouldn’t it just return after treatment? It’s ubiquitous and will even turn up in a dish of water left out for long enough.
This is a ‘take it or leave it’ bit of advice as I can’t explain why it works, just that it repeatedly has. Nothing has been as effective at wiping out BGA as ADA’s Phyton-Git. You try a lot of different products in a store and it came out on top.
It’s supposedly a phytoncide based product and can’t be overdosed to my knowledge. Spot treat the area with the filters off, leave it to settle over the area for 10-15 minutes, siphon out the dead cyano with the water change. Pour in a cap full with the fresh water at water change. It usually rids the tank of BGA within a week, just be keen to remove any dead bits that may be floating around as manual removal is important.
If BGA is below the substrate along the glass you can inject Phyton-Git in with a 1ml syringe. Once dead you will find nerite snails will burrow down to get to it. This has an added bonus that it frees the dead BGA and it may then end up in your filter floss for removal.
Bio security on multiple tanks is not possible from experience. In a gallery of ten tanks with one tank affected, you primarily treat the afflicted tank and put a precautionary cap full of Phyton-Git in the other tanks at water change that week. Practically no one will disinfect their tools so it could potentially be transported system to system.
As with most manufacturers it isn’t really clear what is in the product. Phytoncides are produced by some plants to protect their tissues. It still doesn’t explain how this could be bottled or preserved until the point of use successfully. Did suspect at one point that this may be a red herring, with dormant heterotrophic bacteria being the mechanism at play. However, there is no clouding of the water and glass like you get with the use of other heterotrophic bacterial treatments.
Either way, have found Phyton-Git very effective at eradicating BGA from a system permanently. Also good at tackling BBA and Staghorn on leaves without consequences like with spot dosing excel/glutaraldehyde/liquid carbon. Some will say it is pricey, but a 100ml bottle of Phyton-Git Sol (thicker solution that sinks) lasts years in the fridge. Worth having at your disposal.
Obviously the counter argument is ‘just run a clean tank and you won’t get algae’. However, Cyanobacteria is bacteria and has turned up in tanks at unexpected times for unknown reasons.
Long post about broader experience with BGA, but hopefully something useful to you in there
@dougbraz