but found standard groundbait explodes too much so maybe wet it more and sprinkle some vitalin to make more tacky .
It sounds like you are either using the wrong mi or not mixing it correctly . . . .
Depending on the depth of the water and how long you want the ground bait to take to break down then that will effect firstly the mix you use and secondly the way you mix it.
You can extend the tme it takes to break down by simply usng the one mix and thenL
1. having sieved it a couple of time make up the ball and only squeeze hard enough to make it bind.
2. Make the next set squeezing really hard . . . .
3. Make a 3rd set as per the second set but then wet your hands and put a film of water over the balls.
These will then all break down at different rates over quite some time.
It is a trick we learned on those big bream waters in Denmark where the kiss of death was to keep raining bait down onto the fish. They simply spooked.
By using that method you could keep the fish in your swim for a very long time indeed.
For those Tench I'd probably use something like Van den Eynde Expo and depending on the depth then add in up to 30% Turbo and that should stick like the proverbial to the blanket.
You can always add in some well soaked pellet to eke out the volume and it shouldn't effect the mix too much . . . . .
Have you thought about maybe using a mix of wetted pellet with additional binding agent such as PV1 Collant?
It is a rather sweet but sticky additive than when incorporated in a mix can increase the amount of loose feed you can put in . . .