The use of a top cover has me baffled.
I am re-thinking my methods for a floating top cover in my non-circulating deep water system.
The Net is all over the place on what materials are safe esp polyethelene or extruded types of foam to use in growing hydroponicaly and the sugested n, do not use PVC.
Are there any standards the Kratky method does not and most you Tube clips show all kinds of materials that are questionable.
I would ask for your input on this matter
Norfolk County Massachusetts
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On December 29, 2009 I've posted some Free literature by Dr. Bernard "Bernie" Kratkyfor those who are interested in hydroponics and aquaponics: http://tinyurl.com/pdqt539
Moreover, we have an aquaponics group where lots of information is provided http://www.aquaculturehub.org/group/aquaponics. As far as I know, PVC pipes are used in many of the hydroponics and aquaponics systems and are safe. When using a liner for your growing beds or floating materials it is highly recommended to use "food grade" material. On of the materials that are being used in Hawaii is plywood coated against rotting with paint that is
Please let me know if you need more information.