What is this?

Fenton

Gambusia
MFK Member
Mar 12, 2013
296
1
18
Staffordshire, England
Ignore the lime light seeking Oscar. The black hiar on the to of the plant. What is it? Can I prevent it? Is it bad?

Water, Ammonia 0, nitrite 0, nitrate 5mg/l.

These plants are over 1 year old, been trimmed so they don't stop the lights many times. The plants get around 4 inches cut off every 2 weeks. Just as an idea fast the grow. It's only this type of plant which gets it. Any ideas/advise on what it is?

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