grow beds don't get any solids so already done, except when i place pipe in water and suck out a little manually. how full and flowing should a nft be through a 90mm pipe 6meters long? 1/2 full and water flowing slowly and back into fish tanks is what i thought?

You dont need alot of water in a NFT - just a small trickle is fine as long as you have either a long cup to plant in that reaches down to the water, or you have a wick. As to what effect will it have on the balance? Not much. The extra plants will keep the nitrate levels down. Do you have plenty of filtration (biofilter)?

I'm not sure if I follow you, do you have any pics or plans we can see?
Yes the grow beds are your main solids filter and biofilter (which breaks the poisons down into nutrients). The NFT are empty tubes with water flowing (trickling) through them. In the top of the NFT you drop in some net pots or cups. These are filled with a medium such as perlite. Plants are grown in the cups and the roots will grow through the bottom and into the NFT tubes, and the water will flow over the roots. The problem comes later if you are pumping water from your fishtank directly into the NFT. Your fish water will contain alot of solid organic matter (mostly poo and fish food) which will stick to the roots of the plants in the NFT and eventually form a sticky muddy mass that will choke the plant of oxygen. To prevent this you need to put some sort of filter between the fishtank and the nft tubes to take out all of the solids. This could be a purpose designed filter, or it could be the water coming out of the growbed on its way back to the fishtank. The perlite in the cups will not filter out any of the organic material.