I used 2 X 1000 gallon tanks sourced from a chemical company that made Lecethin. The tanks were not contaminated with chemicals and were easy to wash out, cost me £30 for the 2. Most importantly they are round, square tanks do not work, the eels crowd the corners and drown. The tanks are connected at the bottom with 1 1/2 inch plumbing fittings. A tree of pipework is connected to the outlet in tank 1 with loads of small holes drilled in it. Tank 1 is then filled with beach sand, this acts as a filter as the sand contains the critters and bacteria to keep the water clean.
Tank 2 has a pump, this pumps water through a spray bar over the sand in tank1. It circulates the water and oxygenates it. The water filters down through the sand and eventually finds it's way through the holes in the pipetree and into tank 2 so you have a constant circulation of water.
Put the tanks in a shady place in the garden to avoid the water heating up during the day and cooling at night.
Put the eels in Tank 2 no sand, nothing apart from sea water or the eels can die hidden in the sand and will turn the water. Tank 2 needs to be checked daily as any dead eels left in there will turn the water quickly.
I also have a couple of powerheads in Tank 2 to circulate the water anti clockwise, the eels pick up more oxygen from the water if there is a current to swim against.
You can scale the tank sizes to suit your circumstances and the ammount of eel you want to keep. The best pump to by is the one with a magnetically coupled impeller, no seals to fail. Cost me £80 from the local Aquarium shop. I keep 500 to 1000 eels in this set up.
You obviously have kept sandeels like this for a while. Do you carry out regular partial water changes to keep the salinity up with all the rain we have in Wales, or are your tanks under cover?
I have a smaller setup in the garage, it consist of a 30 gallon plastic filter barrel and a 25 gallon water tank for the Eels. The water is pumped from the water(Eel) tank into the bottom of the filter barrel, at the bottom there is a 1 1/2 inch upvc pipe teed with capped ends and drilled so that the water is sent out through the filter media, the media consists of a layer of cockle shells, plastic pan scourers and fine foam. From the top of the barrel the water enters back into the Eel tank via a gravity fed spray bar, this causes the water surface to be agitated and keeps the air content up. Also at the top of the filter barrel is a much smaller pump, this feeds water through a beer cooler and into the Eel tank. I have always been told to keep the water down to about 10 deg C. I also have a layer of fine beach shell grit and crushed oyster shell in the bottom of the eel tank, but with it being a small setup I can check for the odd dead ones daily. I can keep 2 - 2 1/2 lb of eels in this setup. last year I kept the tank running and kept eels for over six months.
Ps: I have posted a couple of pics of the tanks in the Gallery.