Thanks guys.
I read the articles, and appreciate the link.
Seems I am on the right track but this time I can't drop the ball on it when the biocide runs out!
I will also cut a new bigger hatch in my cabin floor so I can get to my tank easily instead of through an inspection circle.
That will make it easy to just check the tank each year.
I realise that when the boat is on the trailer I can pop the top off the tank and poke my pickup for my siphon/vacumme in the low corner while the boat is on the hill and have any water etc trapped in one corner.
The bug I have is not the black one but a super bright red one that is part of any water layer for sure.
When I looked in the tank with a light I could see the red/water moving on the floor of the tank like mercury runs around a table.
I have it sparkle clean at the moment so now it's ongoing housekeeping and no way around it.