Aquariums: My cycle has crashed.

After 6 months of perfect parameters…..I decided to change my substrate from gravel to sand.

I left some gravel rocks in there along with all the same plants and wood.

But my worst nightmare is happening and my cycle has crashed.

I have low ammonia, low nitrites and low nitrates (which I know that one is normal)

I’ve never done a fish in cycle.

I have 3 electric blue acara cichlids, 5 corydora catfish and a pleco.

I will dose prime. But how do I get my cycle back on track??? Do I add some nitrifying bacteria?

I do plan to do a water change.

Thanks 🙁