Personally I don't see the need to have 2 of each. If you have calibration sol'n and your pH pen nails that then you really don't need a 2nd pen to confirm that the first is good. Make sure you get storage sol'n so the pen can sit in it between uses. I keep a spray bottle full of RO or distilled water handy for rinsing off the probe just before use and again just before sitting it back in it's cap that holds storage sol'n. My pH pen is going on 8 years old now and still calibrates perfectly with both pH 4 and 7. My ppm pen is at least 25 years old and it also works fine. I check it once in a blue moon and it's never more than 20ppm off. If further than that then I replace the cheapo batteries at a bargain store and it's back within 20.
Neither are POS units I got off Amazon either. The ppm pen is a Hanna I paid $75 for back in '82 and the pH pen is a
EcoTestr pH 1 that was $70 at a garden center in Grande Prairie.
I would recommend getting pre-mixed liquid calibration sol'n over the salt packs that come with many pens. The liquid should be as dead on as you can get where mixing up your own stuff can is dependent on the mixer's skills for accuracy and water used to mix with. 500ml bottles of calibration sol'n cost from $5 - $10 each and should last you for years. As they are inorganic salts in water they won't spoil and will still be good long after you're worm food.
What you really want spare of is;
Lights!
Spare bulbs can save your grow. I like top-end bulbs for flowering with Hortilux Super HPS being my all-time fave. Not cheap tho at maybe $90 for a 400W to $299 for a 940W conversion bulb to get massive buds running off a 1000W MH ballast. Only paid $170 for mine 4 years ago but won't need that one anymore as I have a 1000W MH/HPS magnetic ballast and can just buy the regular SHPS bulbs for around $120. I also have cheapo versions of the same kind of bulbs so I can at least keep them going.
Air/water pumps. Running DWC and the air pump croaks? At least have a repair kit for your aerator if not a new pump standing by. With a dual outlet one, one side can die with the other keeps working. When using RubberMaid tubs I run two 12" airstones off one dual pump. The 12" ones lay perfectly in the two slots on the bottom of the tubs and create a great circulation to keep things mixed up well.
Running RDWC? Then a spare water pump is priority 1. Without constant infusion of oxygenated water like DWC stagnation sets in fast and root rot soon follows. Tip: Adding 1ml/L/day of 35% food grade hydrogen peroxide and stirring in saved my DWC back in '02 when we had no power for 8 days and the temps were -20C or lower. I ran a small propane heater, Mr. Heater, with it's element pointed at the plants to give them some light and keep the grow room in the garage from freezing up. Temps were in the low 40s F.
When filled to that ridge the tall tubs hold 50L. The shorter ones hold 35L and the lids are the same size. With 5 or 6" net pots that ridge is just an inch or so under the bottom of the pots which is perfect. I'd veg in the short ones then when I wanted to flip to flower I'd prepare a taller tub with all the nutes needed for a fast switch to flower mode. 3-part nutes at high ppm for Bloom with Big Bud
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Bug spray.
Don't use it just because you have it but have it on hand just in case. I recently got rid of a 2 year mite problem using just Safer's End All with 10ml of canola or neem oil added to each liter I mixed up from the concentrate. The secret to getting rid of them, thrips and many other leaf munching bugs is consistency of sprays. Mites or thrips can be wiped out if you hit the plants every 4th day for 4 treatments. Make sure you get every tiny spot on your plants and even lay them on their sides or turn them upside down to not miss a spot. This stuff is as non-toxic as you can get and a 500ml bottle costs only $15 or so and makes 10L of spray. spray the top of the soil and all over areas the plants sit too. Mites will climb up anything they can and end up all over your Light Rail and crap on the ceiling. I wasn't going to be growing in my basement room for a few months so kept the temp up to 70 after removing the last plants. Mites will go into hibernation if it gets colder than 50F or so. Keeping the temps up with no plants for them to feed on made sure that they would all hatch/die of starvation. I sure hope it hurt real bad!
There's probably more but those are tops for me.
