Might try urethane sealants, they come both no mix and 2 part varieties. 
>From contractor sealant supply houses, check Sika, Tremco, Vulkem as a 
few of the manufacturers. At home depot, it might be sold as a concrete 
caulking, but will be clearly labeled. Cleaning the surfaces is very 
important, generally use a solvent like lacquer thinner. I see that the 
Selley's is not for water immersion. A concern if any of these materials 
are safe for humans and others including fish contact or consumption. 
There are products that are safe for these uses, but needs checking

On 7/17/2010 9:55 AM, ivp wrote:
> One thing I've noticed about silicone is that after a time its adhesion
> to some surfaces isn't so good. It's OK for glass tanks but the joints
> and seals on the plastic materials (PVC and acrylic) on my outdoor
> fish pond weren't as permanent as I'd hoped. Within a few months
> the silicone started to peel at the edges, and I could see water starting
> to creep in. So I switched to this
>
> http://www.selleys.com.au/Selleys-All-Clear/default.aspx
>
> with much better results. They say it's not meant for water immersion
> but I've not noticed any deterioration after nearly 3 years. And the
> sealing part of the joint is not in direct water contact. It's solvent
> based so I had to wait for it to set fully and then flush the system to
> avoid exposing the fish to any nasties. It really sticks and is much
> harder than silicone to remove, yet is just as flexible
>
> Joe
>
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