No such thing as a daft question, especially when it concerns fish's lives
I'll answer your questions in the order they're posted .
1. Nitrite needs to be zero. Your test strips don't include ammonia and that is important as well. Did you cycle the tank with ammonia before getting the fish, and how many plants/what type?
The reason I'm asking questions back is that if you have a nitrite reading now you may also have an ammonia reading, but you can't test for it. Both ammonia and nitrite are poisonous to fish.
Fish excrete ammonia and things like uneaten food decompose to make ammonia. In an established tank (ie several months) there are bacteria which eat this ammonia and turn it into nitrite. Another type of ammonia eats this nitrite and turns it into nitrate, which is less toxic. These bacteria take weeks to grow enough of them to eat all the ammonia and nitrite. The fact that your nitrate is zero does suggest there aren't any bacteria yet to make it.
However, you have live plants, and using plants is another way to to remove ammonia. Plants take it up as fertiliser and they don't turn it into nitrite. But it does depend on the number and type of plant - some are slow growing so they don't remove as much ammonia as fast growing plants - which is why I asked how many plants and what kind.
2. The yellow things. I don't know. Shrimps carry eggs under their abdomens until they hatch, but in some circumstances they can drop the eggs. Egg colour does vary, some are yellow, some are a greenish colour but the 'things' in your photo look a bit bright for shrimp eggs
3. Yes, get a thermometer, either an electronic one or one that goes inside the water. Those that stick on the outside are influenced by the air temperature. If you go for one of those with a liquid inside and you get it from a real shop, compare the temp reading on every thermometer and choose from the group that all read the same.
The thermostat on most heater brands is usually badly calibrated - the water temperature is not necessarily the same as the setting on the dial. It's better to set the heater to whatever dial setting gives the temp you need.
4. The worst thing about 5 in 1 strips is that they don't include ammonia which you need with a new tank. Most of us use the API liquid master test kit
https://www.amazon.co.uk/dp/B00E8B10D8/?tag=
There are other brands, but get a liquid one rather than strips.
Nitrate liquid testers come in 2 or sometimes 3 bottles. One of those bottles contains a reagent which settles on the bottom and the instructions will say to shake that bottle before use, and also the test tube. This shaking is important to get the reagent back into the liquid so follow the instructions with the tester.
5. Fish need less food than you'd think. We need a lot of food to generate heat but fish get their heat from the water.
If there is any food reaching the bottom of the tank, try feeding less, but if the fish eat everything before it reaches the tank floor, that amount should be OK.
6. I would remove as much poop as possible. It looks messy lying there on sand. How is your hoovering technique? I just hover the tube about 1 cm above the sand to clear the poop at every water change.
A couple of other points.
The guppy
may be suffering from ammonia poisoning. Until you have an ammonia tester, I suggest daily water changes of around half the tank volume, then once you have a tester you can be guided by that. Whenever ammonia and/or nitrite read more than zero, do a water change.
Your test strips suggest soft water. This is good for neons but not guppies. It won't be causing the one guppy's behaviour, but long term it won't be good for them. Once they have eventually passed away, I would replace them with soft water fish.
You can check the accuracy of the strips by looking on your water company's website for hardness, though in Cornwall I'd expect it to be soft. Assuming you are with South West Water, enter your postcode here
Our region is divided into a number of supply zones. A Water Supply zone is defined in order to enable the mandatory sampling programme to be undertaken at the correct frequency per populous. Each zone is a geographical area containing no more than 100,000 permanent residents. The results will...
www.southwestwater.co.uk
and look for the German degrees number.
- they play on words has made me smile just when I really needed it today 
