Right, if the bleach smell is there (I'm guessing you didn't go the hydrogen peroxide route, it has no smell) then washing again and scrubbing til the smell is gone would be a good idea. Leaving things in the sun is great for giving a boost to cleaning. I realize that in your locale some things like peroxide might not be readily available but sunshine is plentiful.
If there's no mold, algae, contagion, etc I don't see a reason to boil things. Then again, if that's what you feel is a working plan, go with your gut. The practices that keep this stuff away begin with having a filter, cleaning it regularly, and regular water changes.
As a test, I would try using bottled (not distilled) water for a few water changes, then slowly do a mix with the water you were using, then finally back to your local water and see if the problem returns. If it does, after doing everything else right, I'd say you've isolated the problem as being water-related.
Also, even though I suggested washing the sand, if you're really wanting to absolutely make sure you've done everything possible, consider starting over with fresh sand.