One of the dilemmas of rearing Elysia species with their food algae is providing enough surgy current to keep the algae happy while preventing pumps from sucking in and destroying slow, dumb, floaty, soft sea slugs, or becoming plugged with loose algae.  Here is a simple design that generates surgy current, but keeps slugs out.

The heart of the unit is a small powerhead, with my choice being a Maxi-Jet MJ-600 (600 liters per hour/160 gallons PH).  For a smaller tank, a MJ-400 can be substituted.  The input is screened by a strainer covered by a plastic sponge.  I have found the sponges sold to screen the inputs of Danner Mag-Drive pumps to be about right. Using a small piece of vinyl tubing as an adapter, you can slip a strainer onto the input of the pump, then slip the sponge over the strainer.

Parts of a slug-proof circulation system. The intake of the Maxi-Jet powerhead is covered by a strainer and a sponge, because a strainer alone will not prevent injury to slow, squishy, dumb slugs. The output goes through a Hydor Flo rotating deflector to provide surgy motion.

The sponge is necessary, in addition to the strainer, for a couple of reasons.  First, did I mention that slugs are slow, dumb, and soft?  They will inevitably be pulled into even the small holes of the strainer, causing harm.  The low flow over the large surface of the sponge allows slugs to crawl across it without noticeable pull from the pump.  Also, the larger surface allows debris to collect for a longer time before slowing the output of the pump, so the pump will not need to be cleaned as often.

Surge is generated by a rotating deflector connected to the output of the pump.  I have successfully used the Hydor Flo (pictured at bottom left of the photo above), and the slightly more expensive Innovative Marine Spin Stream.  They both rotate to generate wavy water motion, although the Hydor Flo seems to have a wider flow pattern.  After years of using them, the only complaint I have is that small snails can get stuck in both types of deflectors, causing them to stop rotating (at a frequency that depends on snail density).

Once assembled, the unit is fairly compact, and can be secured by the hanger or suction cups provided by the pump manufacturer.

Assembled Elysia circulator. In the slug system, each one is anchored to the glass with a magnetic holder.

My preferred method of securing the pump is to use magnets, such as those manufactured by Algae-Free, which allows the pumps to be positioned at any desired location or orientation.  In the tank shown below, the pump is placed sideways at a slight downward angle.

10 gallon slug tank a few weeks after system revamp. Sponge-covered circulation pump visible at right. 3/8/17.

In the USG slug system, pumps are turned on and off automatically every 15 to 30 minutes.  This provides plenty of strong water movement, and keeps slugs from being exhausted by constant current.