If your speaking of the Hygger Wave Makers I have both the Nano 1600 models and the larger 2650 models. The Nano wave makers are nice little units once you get an understanding of how to program the controller for your 24 hour cycles. On a 20 gallon long I would for sure recommend (2) of them. Place them about a third way down at each side firing toward one another. One will not give you enough flow due to the long dimension on the tank. The 1600 GPH is only a legit measurement if the program is running at 100% power and in mode 3 which is when the wave maker is constantly on like a regular power head. When you start using modes 1 or 2 which are the alternating wave programs with a ramp up and down feature the 1600 GPH measurement isn't realistic. The Hygger design I like as the ease of maintenance to disassemble and reassemble is easy and they are well built. Honestly if your budget isn't maxed out I would actually go with (3) putting the one of on the back glass in the middle firing to the front of the tank. Amazon runs these for 69.00 each or the larger for 100.00. Take caution buying from them as many times you get one that someone else returned for whatever reason. I ran (2) of the larger ones on my 40 breeder and had them running at 30% of their power and had more than enough flow. I ended up changing them out for (4) of the nano 1600's and put one in each corner and the corals did well as the flow was plenty.
