1st planted aquarium - Cycling CO2 PH advice needed

[plankton]

Continue using the Stability until the cycle is complete.
The plants will use ammonia, but only if there's nothing else available, they will use nitrates first (as far as I understand). It shouldn't stop the cycle, but MAY slow it down.
The problem is that any ammonia or nitrite will upset the immune systems of the fish and may even cause worse problems internally, so that needs to be as close to zero as you can manage, hence the "under 0.25ppm I suggested.

[LadyDay]

you could get yourself a uv steralisor and run it for 3 weeks there about 26 quid on ebay its a internal filter with a uv light built in i got one myself it does the job if u interested i could give u the item number ..

The trade off to using a UV steriliser is that it tends to ruin the chelation of the micro nutrients in the fertiliser, making them unavailable to the plants, so it's probably a good idea to turn it off after the 3 weeks, as you suggest, if he wants to keep a planted tank. Especially since the growth of the plants themselves help combat algae.

[Martinspuddle]

I’m in Wirral. Water generally harder but established tank ph around 7.2 but no co2 and an aerator so am I right in thinking with co2 ph will always be a little lower? Or will it up with cycle progression. Should I aerate overnight to ensure o2 for future fish and rid tank of excess co2 not used that day?! Or is that just wasting it? Pic attached as requested.

You shouldn't need to aerate during the hours or darkness if you have sufficient flow. Excess CO2 in the water column at night as plants are producing their own CO2 can be life threatening to aquatic life because it will lower quantity of dissolved oxygen within the aquarium.