I recently read somewhere

[wwestar2000]

Is it true that they need a specific water condition to stay a HM. Like if you dont change the water for a little more time they will stop being a HM and never be able to be one again. What will it be called then?

 

[LifeBear]

They would be a super delta.

I believe I read that somewhere a long time ago, but I don't know for sure.

 

[Synirr]

I could be wrong, but I don't think the caudal spread actually decreases due to bad water quality... I think you may have misinterpreted the idea. If fry from a spawn with HM parents are kept in imperfect water quality they may never grow to become HMs, but once they have a HM spread I don't think they'll lose it.