Roly, While I wouldn't advise spending money on new gear when you can make what you have work... On this occasion I will.
I cant recommend the Eheim liberties enough for shrimp tanks. Due to the waterfall nature of the returning water it creates high O2 levels which is key for inverts.
The Eheim sponges are great as you can easily pull one out ever other week to rinse and you have the option of having no top on it to fill with emersed plants if you like.
As they are now discontinued, zoo plus is doing them at 20% off. I just bought the biggest one for £20 odd quid to go in shed before they are gone. I'd recommend doing the same as the flow is adjustable and then it is ready for the next nano, not matter the size.
It's a small investment that will pay itself back over and over 🙂