Since you're confident you can keep them, then I'd suggest you get some really fancy substrate. The tank is small so it'll be affordable.
You could get ADA Aquasoil Powder Type or UP Aqua Shrimp Sand (something that's not worth the price if it's for a larger tank). Both are perfectly sized for a tank like that and will buffer well.
Since it's a small tank, you'll absolutely want to cycle things and let it run (by feeding the tank pure ammonia if you don't go the Aquasoil route) for 4-6 weeks to make sure it's suitable for Crystals. That'll be plenty of time to develop nice biofilm and micro fauna.