I hope this works for you. I have a picotope that I drilled and made a 2.5 gallon sump and 2.5 gallon ATO for and STILL had a difficult time with it. SW tanks are very difficult to keep stable in such small sizes though not impossible. Just be prepared to do regular maintenance and remember to use FW for you top offs NOT SW.
Also once you get fish in it, you can easily keep the water params good by changing small amounts of water each day. I always had a small amount of pre mixed SW ready and simply changed a few cups of water a day. Took only about a minute and kept things happy. No need for big water changes and wild swings this way.
Small amounts often with a gentle wipe will go a long ways to making this work.