Fluval Cycle.

There are many bacterial starters on the market; most are regarded as useless. Several reasons.
The manufacturers may claim that their product has spores of the bacteria - but the filter bacteria do not make spores.
Some contain the wrong species of nitrite eating bacteria.
Some contain dead bacteria.

When these starters first appeared on the market it was known that the ammonia eating bacteria are Nitrosomonas species.
But it was thought that the nitrite eaters were Nitrobacter species because of work done in water with high ammonia levels, then they applied this research to aquariums. Dr Timothy Havonec discovered that the correct nitrite eaters in aquariums are Nitrospira species. Both Dr Havonec and the company he worked for copyrighted or registered (or whatever the correct legal term is) the use of Nitrospira. Dr Havonec set up his own company which produces Dr Tim's One and Only (though it doesn't seem to be available in the UK) and the company was taken over by Ttera which markets a modified version called Tetra Safe Start. These are the only two products which contain the correct species of nitrite eaters.
And even these two products won't work if they have been stored incorrectly at any time between the factory and your tank. If the temperature is too warm or too cold, the bacteria will die.

If you want to try a starter, look for Tetra Safe Start. I have not used it myself (I have only done a fishless cycle without anything but ammonia) but I think it comes with fairly specific instructions, and they probably only talk about using it with fish in the tank. Use a combination of their instructions and ammonia instead of fish. You may find a bottle that has been stored OK and if so the cycle will go faster.

The product made by Hagen called Cycle is usually compared to snake oil.

Snake oil is an expression that originally referred to fraudulent health products or unproven medicine but has come to refer to any product with questionable or unverifiable quality or benefit.

 

I've used Tetra Safe Start for a couple of weeks each time I've put a new shoal of fish into either the main tank or the quarantine tank and also when replacing parts of the filter media.  It's certainly never done any harm and therefore I would continue to use it in this way for my own peace of mind even if there is no scientific evidence to back this up, simply because I've been relieved that all seems to have gone well when I happen to have used it.