So i removed the pebbles and did 50 to 60% water changes for the next few weekends, both sat and sun. Probably about 6 of those water changes so far. I bought a 120l barrel and a pond pump, i used water conditioner recommended and get the water to the right temp in the barrel before pumping into the tank and its been going ok.
I purchased some while aquarium sand and that's now in the tank. I have managed to get the nitrate levels down to 25 to 50 mg
/l for about the last 12 days. I've added some Elodea Densa plants and the nitrate doesn't seem to be getting any lower. I know zero is best but are my levels ok for a tropical tank. Before i joined the forum my nitrates were higher than 500 on the test strips so it is a massive improvement. The water looks much cleaner, though i wouldn't say it looked dirty before.
Thanks for the helpful advice that you have all given.
I'm not sure if i should keep doing the 60% water changes or just wait and see if the nitrates lower now i have the sand and plants in the tank.
Thanks again all.