First time hydro

Lookin' good!
You're going to need a much bigger air pump soon.
And bucket...

 

No, larger capacities of water/nutes help stabilize it for longer periods. 
pH fluctuation can be from a number of things.
Anything from temperature changes, nutrient level changes from uptake, to microbial activity, and on.
 
Edit: Don't know if you've switched to proper nutrients yet, but those sticks could become an issue.  

 

You don't need a large amount. You need a balanced amount. 
Without balanced amounts of nutrients, you'll always have pH shifts.

 

Exactly.  

 

Without some proper nutrients soon, I think you're going to run into micro nutrient deficiencies.
 
There's a big difference between hydroponic nutrients and soil amendments.

 

Looking pretty good mate! But I would suggest you get a bigger bucket. From what I can see you could probably fit a 3 - 4 gallon bucket in there and you will definitely need it later on, that plant will soon outgrow that 1 gallon(?) bucket. You can buy a larger black bucket with a lid on eBay for about £5 - £10 and then you won't have to worry about getting it quickly when you realise that the roots have outgrown the small bucket.

And I agree with everyone else, you definitely need some real hydroponic nutrients, I know you might not have a lot of money and I understand that, but that plant won't survive forever without decent nutes, be it General Hydroponics, Canna, Dutch Pro, or any well known companies.

The best thing you can do is look online/on eBay and find a part A and part B nutrient system, they're probably the cheapest and easiest way of using nutrients.

http://pages.ebay.com/link/?nav=item.view&alt=web&id=321591408662

Here's a link to a Dutch pro "Grow A and B" and "Bloom A and B". £26 for both and that's the base of what you need, you could do with buying some other things but they should support your plant up to harvest. You could even just buy the Grow A and B for the moment as you're not flowering and then buy the Bloom later.

But other than that it's looking good man, your plant doesn't seem to be showing any bad signs! Looks much better than the start of the thread

 

The directions say 1/4 teaspoon per gallon for constant feed, man.
I'd dump 3/4 of your gallon out, and replace with pH'd water ASAP!
See how it does for a few days, then move up to the 1/4 tsp per gallon.
Also, you need to beg, borrow, or... something, to get the biggest air pump Walmart has. Maybe even two.

 

Get decent nutrients and you will never have to check PH. I have a bunch of hydro plants hitting the ceiling growing hog wild and its been weeks since iv checked PH.
 
When you make soup dont even bother to PH it till its been a couple days unless you know you will have to add some ph down or whatever but dont go make the soup 5.0 thinking it will climb to 6.0. Sneak up on a PH shift, the scale is logarithmic not linier.
 
if you have decent water to start with that is key. Crummy water will give the very best growers fits.
 
My water comes out of the tap @ 40PMM 7.1PH

 

Yeah, I wouldn't say those are proper nutrients for hydro.
Better than those sticks I suppose.
I wish I had good tap. 
 
No idea what's in it, and it smells worse than a public pool. 

 

You can make your own organic hydro juice with bloodmeal, fishmeal and liquid seaweed. Most or the organic hydro nutes Im getting are molasses based.
 
The plants love the General Hydroponics Floronova Bloom, 2TB per gallon. 1 CC of cal mag per gallon and the plants are super happy. picture perfect plants with no issues.

 

 
The plants are always right, if they are happy then run with it. Its nice to get some you know and trust and stick with them. I hate to switch when I know some that work..