mold on wood aquarium cover
 

Hi, I have a small 20g aquarium (13 in front, rest is ATO and built-in overflow and nano octopus skimmer).

The hood is wooden and I just noticed growing mold growing on the inside rim. Its had water in it about a month and a half.

How can I protect the wood to prevent the mold? I tried searching but to no real available despite finding on a post on the net regarding varnish. I was just worried the varnish could seep into the aquarium though.

What would be the best way to seal the wood and prevent this?
Would siliconing the underside work?

Kill the mold first by using a toothbrush and scrubbing with a bleach/water mixture. Allow to completely dry. Then use an acrylic wood sealer to prevent further mold growth. I've used sealer products by both Zinsser and Killz to good effect in my hood and stand.

Thank you for the info, very much appreciated. I just didn't want to use anything with anti-mildew that would leech in.

Use an outdoor grade sealer or finish
 

If you don't use the Zinser specifically, you can use any outdoor grade product. Any kind of deck sealer should work. They all have UV resistance, and contain moldicide and fungiside.
I don't think it should be a problem if your hood is ventilated, but you may want to wait long enough for the canopy to completely dry (days as opposed to hours) for the volatiles to dissipate.