Yup, sexually maturity. Axolotls can come into sexual maturity any time from six months to two years after they hatch- My original axies were hatched june/july last years and were showing signs of maturity back in Feb/March/April, though the longer you can leave off on breeding the better (Well over a year is usually a good guideline, but sometimes they have their own ideas and eggies end up covering everything-This has yet to happen to me, but considering Bertie appears right now to be my only female, not too surprising).
Glad you're getting rid of the pebbles- If you're having trouble getting a new base, go bare bottomed for a while, it won't make that much of a difference and will save you the stress of freaking out over if he's decided to geologically supplement his diet. There are some really pretty sand mixes if you want to keep it colourful. As to telling if he's swallowed one, keep an eye on him. Search on the site for impaction, watch if he starts floating (Not just hanging out on the filter cables and plants, but spaceship like). Treatment if he has swallowed one is usually fridging- there are some excellent resources on how to do it, as they cold temps of the fridge slow down the metabolism and make passing things much easier on the critter.
As to water parameters, you'll need to pick up a DROP test (Not a strip test!) Master kit- They're between $40 online and $200 in the shops- the ultra expensive ones are for those who like to go overboard (Eventually I may even get the overboard one myself
) Also, a thermometer to make sure your tank is below 23c (Above that, things start getting hairy and deadly...).
Hope I've been able to help (And I'm sure everyone else will chime in in a few minutes!)
