I was diagnosed with old tank syndrome on another thread. High ammonia low ph high nitrates no nitrite. I have gravel, plants, small driftwood, rocks in my 10 gal. 2 mollies two KuhlI loaches. I did the recommended big change, 60% one day and 30% the following two. Only raised ph a little and lowered others a little. I tried to deep clean my substrate but couldn’t get it all. So, I got some CaribSea supernaturals sand, if I did a 60% td and changed substrate completely would my tank cycle or be ok? (Rinsed my filter in aquarium water 48 hours ago and added my first diy media to topfin hob. Old filters still in there to spread bacteria) . I will put fish in small bucket for a few hours till it settles. PS I made beginner mistake and added these fish in the day I set up my aquarium a year ago and they all lived so there toughies.