Tank For A Betta

It's fine for a betta. Yes they do prefer shallower tanks as it is less distance to swim upwards for air. Those fins weigh them down. The volume is fine as well. The recommended minimum is 5 American gallons which is 19 litres. For that sized tank a 25 watt heater would be fine, though you might have to have it at a 45 deg angle to make it fit.

I've had Clearseal tanks in the past and they didn't come with lights - does yours have a light? It's only really an issue if you want live plants. If you want synthetic, get silk plants not plastic as they can have sharp edges. Decor needs to be smooth as well - use the tights test. That's just an old pair of tights rubbed gently over the decor to see if they snag on it.
Substrate can be anything you want from glass beads to sand.

Feed pellets food designed for bettas. Flakes can cause bloat. Soak the pellets first so they are already swollen up before the betta eats them. Live or frozen live once or twice a week. Avoid dried bloodworm as they are well known for causing bloat. And remember that bettas are pigs when it comes to food. They will beg for food whenever they see you. Don't give in!

Actually, I've been looking at 25W heaters on the internet. There are some horrifying reviews where customers have lost fish due to the thermostat over- or under-heating the water, or the heater exploding. Makes me a bit nervous. Does anyone have any knowledge of heaters that are reliable and trustworthy? Ideally one with the numbers displayed and a light to indicate when its on would be good.

Great. thanks Sue, will investigate one of these then