Ah ok, this makes it easier because I have a Rio125 too
Lighting sounds ok. The Juwel reflectors will help intensify the light towards the tank.
CO2, I really would try get some carbon to the plants. Even with the 2x18W you have in there the plants are going to need carbon in order to grow and maintain their structure. If you don't want to go down the CO2 gas route then there is another option. You can introduce carbon via a liquid solution called
Easy Life EasyCarbo. Dose 5ml of that a day and you will see a positive effect. Also, don't use the airstone as it is just driving off CO2. Airstones IMO are just there to look nice. In a heavily stocked aquarium with large cichlids would need a air pump, but for your tank, it's not necessary.
Another filter would be ideal to create more flow and offer more nitrifying bacteria however you could just opt for a powerhead to add flow. Good amounts of flow are needed in a planted tank to distribute nutrients and CO2 to the plants and the plant's waste away from the plants. The
Hydor Koralia Nano Pump would be a good choice. I'd position it on the left hand side of the tank, near the front and just below half way down.
Water changes need to be increased more. Atleast try do it every week of about 30% (50% would be much better). This way you effectively reset the tank, stabilize the KH and pH, aswell as reducing the ammonia and algae spores in the tank.
Try and clean the filter floss (the white floss at the top of the filter) once every 2 weeks. This will get rid of any organic material that could be leaching ammonia that feeds algae spores. The cleaning of sponges sounds right. It's a good idea to siphon out the bottom of the filter to get rid of the mulm down there too.
Nutrients. Again, the plants really do need this, any gardener will tell you that. Best one for you is the
Tropica Plant Nutirtion +. (the "+" is important). Aswell as containing the trace elements for the plants it also contains macro nutrients aswell. Marco nutrients being nitrogen and phosphorus. Contrary to what you've been lead to beleive, these do not cause algae in a planted tank (if of course everything else is balanced). Try dosing 2-3ml of this a day. If the plants look like they need more, then increase by 1ml.
When you've got all you need, then go get some more plants and watch the magic