Its very easy to wonder accidentally into higher light scenarios without realising it.
My mate has a tank with a single T8 tube, tank maybe 25years old now (steel framed, remember those) in which he keeps a few fish and slow growing plants, generally replacing plants every couple of months as they don't really grow just stay steady. Never had any algae issues.
One day the tube ballast failed, possible due to water ingress along cables from 25 year old crumbling T8 tube holders. Decided to replace with equivalent length T5 tube, got a couple of water proof tube holders from Ebay and T5 tube and ballast from electrical supplier. Fantastic tank was much brighter, plants went ballistic, cutting away every week...then it all went pear shaped, as I assume the plants scoffed all the food supply, melted away and algae started appearing on plant remains, glass hardscape in fact every where.
Quick consult me and verified he had moved from low light to higher medium which explained his issues.
Sorted by turning light period down, frosted piece of plastic on top of tank cover sheet to reduce light level and 1/4 dose dosing liquid carbon and 1/4 dose EI ferts (borrowed from me 😀). Quite chuffed as didn't cost a lot to fix (£9.99 for carbon, free fertz). Anyway no algae seen, plants now grow a bit, compared to T8 tube and tank looks much better in brighter light especially the green plants.
