Aquarium Silicon 509 ASI

ASI 509 Aquarium Silicone (sometimes written as “Aquarium Silicone 509 ASI”) is a professional-grade aquarium-safe RTV silicone sealant used for building and repairing fish tanks.

What it is

• A 100% RTV (room-temperature vulcanizing) silicone
• Designed specifically for aquariums, terrariums, and glass panel sealing
• Cures into a flexible rubber-like waterproof bond

Key features

• Aquarium safe (safe for freshwater & saltwater once fully cured)
• Strong adhesion to glass without primer
• High strength + flexibility (handles water pressure and tank movement)
• Non-slump formula (won’t sag on vertical seams)
• Clear or black options
• Resistant to long-term water exposure and yellowing  

What it’s used for in aquariums

• Sealing aquarium corners and seams
• Rebuilding or resealing tanks
• Installing or repairing baffles in sumps
• Bonding glass panels in custom tanks
• General reef tank repairs

Important notes (this matters)

• Must be fully cured (usually 24–48+ hours, sometimes longer for thick beads) before adding water or livestock
• Old silicone must be completely removed before resealing (new silicone doesn’t properly bond to old cured silicone)
• Avoid contamination (oils, dust, wet glass)

Why reef builders use it

It’s basically considered a “pro builder” silicone—the same type used in manufacturing aquariums, not just DIY repair kits.