Not sure how much extra space you got in there but you might be able to build a wooden frame around the aquarium before you silicone it. Really as small as it would be, you prolly wouldn't even need any reinforcement. As far as filtration goes, maybe a little corner sponge filter? I think simple is the key here. Since this post is already almost a year and a half old, maybe you already have your second generation of micropoecilia in there, and if that is the case, I wanna see some pics!