How far in and how wide is the tunnel? The needle that comes with Aptasia X is about 2" long, and the syringe is fairly Narrow, so it would fit in a small-ish tunnel. If you can see the Aptasia, then it must be coming out at some point. Just get it when it comes out, then when it retreats, fill in the whole width of tunnel if the syringe won't fit. Don't do the whole length of the tunnel, just across it.