What you have read is not quite correct. You are going to see gains, little losses and maintains and up to 50g either side is well within normal fluctuation and definitely does not require you to change anything.
A young piggy can gain quite a bit in a week - if you are feeding a good healthy balanced diet. As your piggies appear to be around the start of their teens you should be seeing a good upward trend towards their adult weight.
Once they reach their adult weight then they will
start to maintain but you will still see small losses and gains within that. Provided it’s roughly stable - up to 50g either side of their regular weight - then that is good and what you are looking for.
Any losses less than 50g are not concerning (unless they form part of a longer downward trend) - up to 50g loss is the difference between a full or empty bladder or tummy. You don’t need to change anything if the diet is a healthy balanced one.
A full bladder can weigh 10g and a full tummy can weigh 30-40g so this is why up to 50g loss is not concerning - could just mean you’ve weighed them just as they’ve had a nap so not eaten for a bit, and done a big pee so they weigh less for a little while
If losses are less than 50g a week but they form part of a longer trend over several weeks but is generally going down, then you need to offer support feeds and see a vet.
Any sudden loss more than 50g, particularly if that happens in less than a week, require you to step in with syringe feeding and seeing a vet.
100g loss in 24 hours is an emergency.
Provided you are feeding an unlimited hay diet, one cup of leafy veg (not it’s sugar items such as carrots, fruit) and just one tablespoon of pellets per pig per day, then there is nothing that needs to be changed in the diet.
The other thing you need to check is their heft - piggy BMI.
While weight tells you that they are eating enough, weight does not tell you that they are a healthy size for themselves. You need to check their heft to do that. The guide below explains how but basically you check around their ribs and make sure you can just feel the ribs with a thin fat layer.
If piggy has an unhealthy heft ie you cannot feel the ribs at all, then you need to look and potentially make changes to the diet. You may be feeding too many pellets or too much carrot and fruit. They may also not be getting enough exercise.
Weight - Monitoring and Management
Long Term Balanced General And Special Needs Guinea Pig Diets
This guide also talks about weights throughout life
Journey through a Lifetime: The Ages of Guinea Pigs
