Capillary bonding is the most well known method for joining acrylic. And from the number of posts, easy to assume it is the only way. As in all methods, surface preparation is key to success. For this article's purposes, I will assume properly prepared surfaces are the case and the solvent we are using is un-thickened Weldon 3.
Pins or no pins. Placing pins between the surfaces to be bonded creates a slight gap making it easier for the solvent to travel by capillary action. For smaller parts and short lengths pinning is usually not necessary. For longer lengths and thicker parts, pinning is desirable. Pins should not be too thick or too thin. Thick pins such as those made from paperclips create a gap too wide for capillary action. Too thin and you risk breaking them during removal. There are many good online sources that discuss some rather clever pinning methods.
Maximum piece thickness. The maximum material thickness using pins should not be greater than 1 inch unless the parts are small (easily picked up with one hand). Aquariums built with acrylic thicker than one inch usually are taller than 48 inches and considered to be on the larger size. Even using acrylic instead of glass, they are heavy and require special methods to move. Pinning these large sheets is not advised for two main reasons:
1) Solvent cementing is not the strongest bonding method for these large tanks. While it is technically possible to achieve good seams, there are better, stronger ways to bond thicker gauges. I will discuss these options in a future article.
2) The pins will be buried in the panels as the solvent softens the surfaces. The considerable weight of the top piece will bury the pins making them difficult to remove. And if a pin breaks inside the seam, be prepared costly and time consuming rework.
For smaller pieces such as bases, ornamental fabrications and even rimless frag tanks, pinning pieces thicker than 1 inch is acceptable. But again, there is no reason for using such thick and costly sheets for smaller builds.