In that case, don't worry about the pH. It just occurred to me that the water in the bucket with the redmoor wood could have fallen because that's what your water does on standing, without redmoor wood. Since your tap water doesn't change on standing, it's the wood dropping the pH in the bucket. The effect should diminish over time. But it does mean that it is likely to drop the pH in your tank, so doing a big water change would alter the pH in the tank quite a bit - smaller, more frequent water changes would be better.
The gobies will breed in something bigger, they tend to like small entrance holes. If you could find something to reduce the entrance hole to almost the width of the fish, they'd be happier. With these fish it is dad that looks after the eggs, so the holes needs to be big enough for the male to fit through.
Read up on how to tell the difference between males and females - the best way is the shape of the head rather than colours on the edges of the fins, though bear in mind that the 'bulgy' head only develops at puberty and fish in the shop are usually youngsters. You need one male (or two males if the floor if the tank is easily divisible into two territories) and at least one more female than males. I mean 1 male and 2 or more females; 2 males and 3 or more females.
My cycle isn't going
It got stuck. 5 days with no change in ammonia or nitrite. I started with 2ppm ammonia as the tank the filter is going in has only 1 fish and 1 snail and that was probably way more than it needs. On day 16 I had ammonia between 1 and 2 and nitrite under 0.25 (just the merest hint of purple, I had to do a test on tap water to see the difference). Day 19 exactly the same. Then I got impatient and tested on day 21 instead of day 22 and got exactly the same again. There is no substrate in the tank and there were big white flakes and tiny brown bits all over the bottom. So on day 21 (Tuesday) I emptied all the water, refilled and added 3ppm ammonia (so it fits in with the levels in the method). This is quite easy when your tank only holds 24 litres. Next test day is tomorrow.
I last did a fishless cycle 3 years ago using the old method, and the ammonia reading fell on day 12 - which is why I'm getting impatient this time.
I'm still wondering what all the bits were
One thing does worry me a bit. This is a sponge filter powered by a brand new air pump. The airline tubing is turning yellow where it leaves the pump. Is there oil residue or some such leaving the pump and making its way into the tank and when it meets water turning into the brown dust? Though I suppose that by the time this cycle eventually finishes, all the 'loose' chemicals from the pump will have gone.
I usually seed a new filter using old media but that is not possible going from an internal with a rectangular sponge to a filter which uses a specially shaped cylindrical sponge. I could have rinsed out some dirty media from another tank in the water to add a few bacteria and hopefully speed things up slightly, but it is good for me to see what new fishkeepers, with no access to old media, go through