It seems a good idea to replace the sand. A lot of work, I know, I have replaced the gravel with sand substrates in five of my aquaria a year or so ago. I use Quikrete Play Sand from Home Depot. At $8 something a 25kg bag it is very inexpensive, and it replicates the sands in the Amazon. From your photo I am assuming the pool sand you used is white; it is best not to use white sand for reasons of the fish and aesthetics (viewing).
OK, this settles the ammonia issue. This level is not bad, frankly I wouldn't worry about it. But I would recommend a water conditioner that detoxifies ammonia so there is no sudden influx at water changes. Most achieve this by changing ammonia to ammonium, which is fine.
So back to one of the initial questions on cycling...with this info, nothing seems to be going wrong in your tank with nitrites/nitrates.
This is fine with soft water fish. Depending what you do re plant fertilizing (the "hard" mineral issue I referenced previously) and water changes (I do half the tank every week) it will or should stabilize.
This may have been bleach from the sand, as the pepper are fairly hardy corys. The julii are likely not Corydoras julii but C. trilineatus which is what I see around here labelled julii. But that doesn't matter, both are nice little corys. If they really were C. julii, they will be wild caught which means care with water parameters especially during acclimation.
These numbers are fine. As I said previously, ammonia is ammonium so not an issue and this will likely sort itself out. I still feel some of this is from the bleach residue and new sand will deal with this.
Let me know if I can offer further.
Byron.
I will say that before I went for vacation, I vaccumed the sand really good, (approx. 40% water change), filled the tank and "primed" it. 
