You'll see a lot of debate about CO2 online. Here's the straightforward version.
For the epiphytes we recommend to most beginners — Anubias, Java Fern, Bucephalandra — CO2 will help them grow better, just as it does with most plants. However, because epiphytes are slow-growing, they don't consume nutrients quickly. If CO2 and nutrients are out of balance in the tank, algae can take hold before the plants have a chance to use those nutrients up. It's not that CO2 is bad for epiphytes — it's that it needs to be introduced as part of a balanced setup, not just added in isolation.
For rooting plants like Vallisneria, Cryptocoryne, and Echinodorus, CO2 isn't essential but will improve growth and colour. Good lighting and root fertiliser will often get you a long way without it.
For stem plants and more demanding species, CO2 makes a noticeable difference. But even here, you have options before committing to a full pressurised system. Liquid carbon is a good starting point — it's more useful for keeping algae in check than as a true fertiliser, but it does help. And if you have a smaller tank and want to try CO2 without a big outlay, disposable CO2 kits cost under £15 and will last 30 to 60 days depending on use. They make a real difference to plants like Monte Carlo that struggle without it.
The honest truth about plants is this: to get the very best from them, you need the right balance of light, water quality, and nutrients. The best equipment can be expensive, but there are options at every budget, and you can still create something genuinely beautiful with modest outlay — if you choose the right plants for what you have.