I haven't done this.... BUT.... I do know that kitchen/bathroom silicone contains anti-mildew chemicals that can and will leech into your water and WILL affect the inhabitants. Regular 100% silicone is available at Canadian Tire, Home Hardware etc... and is clearly marked "Ideal for Aquariums".... available in white or clear... if you want black you have to get industrial stuff... I don't have it in front of me right now, but I did post the product # in another thread asking about this...
I'm not going to say that is your problem... just my two bits on the silicone. One thing I ask if your Xenia is receeding is how are your iodine levels? Another suggestion is the first fix... try doing some water changes... if there are any leeching chemicals, they would be somewhat diluted this way...
I hope you find the problem... 
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135G Mixed Reef. Bullet 2, 25 gal refugium, 2 X250W MH + 4X 96W PC\'s, DIY Calcium Reactor, Coralife 1/6 HP Chiller, Phosban, Tunze, 2 closed loops & SQWD\'s, Seios, Coralife 4 stage RO/DI & a bunch of other expensive gadgets... I may never retire, but I'm gonnahavahelluvanaquarium!
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