Fresh water tank slime
Dave, the first question that leaps to mind is ''where do you get your water''? Before you say the city water''s fine, ask around the marina about any other folks having this problem. Marina water delivery systems can be a big problem.
I''d recommend you a) eliminate the existing problem, and then b) guard against reintroducing it. For a) I''d close the tank supply line, fill the tank with an agressive clorine bleach/water mix, then carefully top up the fill hose with the same mix to the very top (which will also drive the mix up your vent hose). During the ''fill'' process, run sufficient water thru your hot water heater (if equipped) and both H & C taps of each fixture so that all your supply lines and heater tank are also being purged. Let it sit most of a day, then repeatedly flush, tank empty/tank full/tank empty. (BTW don''t do this if you have an aluminum tank, which you probably don''t). Flush using every fixture, hot & cold taps. This assumes you''ve got the tank scrubbed as best you can before beginning; dead bio organisms aren''t going to leave the tank but rather just sit there and float disgustingly around.
For b), consider using an activated charcoal 5 micron filter system hooked up to your dock hose, available at Lowes/HD for perhaps $40. And of course, continue treating the water altho'' that won''t prevent mini-blobs from being delivered to the tank. After filling the tank a time or two, extract the (reuseable) filter and examine it; what are you seeing that otherwise would be going into your tank? If it''s pretty visible, I''d recommend showing it to the marina staff. That''s a great way to objectively demonstrate they may have a line breach somewhere or are taking on bio-polluted water from their supplier.
Good luck with Blob Eradication!