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.

Aquarium Silicon 509 ASI